sslutil: eliminate `modernssl` by constant-folding code using it

hgweb: avoid using `sslutil.modernssl` `sslutil.modernssl` is going to be removed. Since the point of using this attribute was to check the importability of the `sslutil`, a different attribute can be used. `sslutil.wrapserversocket` is used because it’s anyway used a few lines below.

sslutil: remove comments referring to removed SSLContext emulation class

sslutil: remove code checking for presence of ssl.SSLContext Now that we require the presence of ssl.SSLContext in setup.py, we can remove this code.

setup: require a Python version with modern SSL features This increases the minimum security baseline of Mercurial and enables us to remove compatibility code for supporting older, less secure Python versions.

sslutil: set `_canloaddefaultcerts` to `True` if `ssl.SSLContext` is present The `load_default_certs()` method was already present when `ssl.SSLContext` was backported to Python 2.7 (https://hg.python.org/cpython/rev/221a1f9155e2).

filemerge: add __bytes__ for absentfilectx This will at _least_ aid some upcoming debugging. Differential Revision: https://phab.mercurial-scm.org/D8592

mergestate: move staticmethod _filectxorabsent to module level I suspect this was a static method just because it made merge.py feel less messy, but now we have a mergestate package so we can do better. Differential Revision: https://phab.mercurial-scm.org/D8591

rust: remove support for `re2` With the performance issues with `regex` figured out and fixed in previous patches and `regex` newly gaining support for empty alternations, there is no reason to keep `re2` around anymore. It's only *marginally* faster at creating the regex which saves at most a couple of ms, but gets beaten by `regex` in every other aspect. This removes the Rust/C/C++ bridge (hooray!), the `with-re2` feature, the conditional code that goes with it, the documentation and relevant part of the debug/module output. Differential Revision: https://phab.mercurial-scm.org/D8594

rust-dependencies: update `regex` to 1.3.9 Version `1.3.8` introduces support for empty alternations, which makes previously disallowed patterns usable in `regex`. From a user's perspective, this means that glob patterns like `*.py{,c}` will no longer generate an "invalid" regex and will use the Rust path. `1.3.9` is a bugfix release, might as well update to the latest one. Differential Revision: https://phab.mercurial-scm.org/D8593

cleanup: remove compatibility code for Python < 2.7.4 The minimum supported Python version was recently raised to 2.7.4.

cleanup: eliminate procutil.quotecommand() After some compatibility code was removed, the function was the identity function on all platforms.

cleanup: remove compatibility code for Python < 2.7.1 The minimum supported Python version was recently raised to 2.7.4.

grep: reduce the cost of pathauditor checks when grepping working copy Running `time hg grep zxczxczxczxczxc -l` on mozilla-central: before: real 0m20,000s user 0m15,796s sys 0m4,189s after: real 0m10,903s user 0m8,964s sys 0m1,916s if vfs didn't call pathauditor at all: real 0m7,781s user 0m5,968s sys 0m1,790s Differential Revision: https://phab.mercurial-scm.org/D8582

grep: test that paths get audited Differential Revision: https://phab.mercurial-scm.org/D8581

grep: add test coverage of behavior on symlinks Differential Revision: https://phab.mercurial-scm.org/D8580

help: fix description of revlog version 2 Differential Revision: https://phab.mercurial-scm.org/D8576

files: speed up `hg files` when no flags change display It's not the first time I see slowness from this command slow down tools built on top of hg. The majority of the time is spent merely printing the result before this change, which is clearly not how it should be (especially since the computation of the result also looks slow). Running `hg files` in mozilla-central: parent revision: 1,260s this commit: 0,683s this commit without batching ui.write: 0,931s this commit replacing the body of the loop with `pass`: 0,566s This looks like a prime candidate for a rust fast path, but until then, it seems reasonable to optimize the python. Differential Revision: https://phab.mercurial-scm.org/D8586

sshpeer: make client print (likely) server errors on stderr (BC) so `hg clone -q` or `hg pull -q` don't print `abort: no suitable response from remote hg!` with no indication of what went wrong. There are other errors still silenced by -q (like failing to push due to a server hook), but the current change covers a good fraction of the problem (all errors setting up the ssh connection, no such remote repository, no access to the repository). Differential Revision: https://phab.mercurial-scm.org/D8584

sshpeer: add test showing that -q silences remote errors Differential Revision: https://phab.mercurial-scm.org/D8583

scmutil: speed up relativization of paths when it's a no-op Running commands from the root is commmon, in particular for automation. Running `hg files > /tmp/a` from the root of mozilla-central on linux: before: real 0m1,510s user 0m1,387s sys 0m0,090s after: real 0m1,266s user 0m1,165s sys 0m0,073s (there are 280k paths, so this was costing ~1us per path somehow) Differential Revision: https://phab.mercurial-scm.org/D8585

context: implement mergestate() method This will let us have the mergestate storage be controlled by the context. In particular, for working contexts we should use the existing mergestate, but for overlay contexts it's inappropriate to drop files in .hg/merge. Differential Revision: https://phab.mercurial-scm.org/D8551

mergestate: split out merge state handling code from main merge module There's already some pretty reasonable encapsulation here, but I want to make the mergestate storage a property of the context so memctx instances can do a reasonable thing. This is the first step in a reshuffle to make that easier. Differential Revision: https://phab.mercurial-scm.org/D8550

tests: add coverage for repo.changelog.children() in the git extension Differential Revision: https://phab.mercurial-scm.org/D8548

tests: add coverage for repo.changelog.findmissing() in test-git-interop.t This at least does a basic test of the method. It's not super-complete, but it's better than the nothing we'd otherwise have. Differential Revision: https://phab.mercurial-scm.org/D8547

relnotes: add API change note per request in D8502 Differential Revision: https://phab.mercurial-scm.org/D8549

merge with stable

grep: grep the working copy faster `hg grep qqqq` in the mercurial repo: before: 0,859s after: 0,233s `hg grep somethingwithnomatch` in mozilla-central: before: 51s after: 19s This is probably also a tiny bug fix, because the code was looking up a node for filename `pfn` on a filelog for filename `fn`, which are most of the time the same filename, but don't have to be. Ignoring performance and the bug fix, the code should have the same behavior. Differential Revision: https://phab.mercurial-scm.org/D8545