subrepo: activate clone pooling to enable sharing with remote URLs This is the easiest way to ensure that repositories with remote subrepo references can share the subrepos, consistent with how local subrepos can be shared.

subrepo: don't attempt to share remote sources (issue5793) Untangling _abssource() to resolve the new subrepo relative to the shared parent's share path, and then either sharing from there (if it exists), or cloning to that location and then sharing, is probably more than should be attempted on stable. Absolute subrepo references are discouraged, so for now, this resumes the behavior prior to 68e0bcb90357 of cloning the absolute subrepo locally.

test-subrepo: demonstrate problems with subrepo sharing and absolute paths This affects remote paths in .hgsub, as well as clone pooling from a remote source. For reasons unknown, there are stability issues with the relative-path.t tests. If run as a single test, it is stable. If run with --loop, or with -jX for X>1, the hash of the parent repo changes. I'm seeing this on both Windows and Fedora 26. I added an `hg log --debug`, and the manifest hash changes, but I have no idea why.

annotate: do not poorly split lines at CR (issue5798) mdiff and lines(text) take only LF as a line separator, but str.splitlines() breaks our assumption. Use mdiff.splitnewlines() consistently. It's hard to read \r in tests, so \r is replaced with [CR]. I had to wrap sed by a shell function to silence check-code warning.

setup: only allow Python 3 from a source checkout (issue5804) People are running `pip install Mercurial` with Python 3 and that is working because not everything performs a Python version compatibility check. Modern versions of pip do recognize the "python_requires" keyword (https://packaging.python.org/tutorials/distributing-packages/#python-requires) which we set if using setuptools. But this isn't set nor recognized everywhere. To prevent people from accidentally installing Mercurial with Python 3 until Python 3 is officially supported, have setup.py fail when run with Python 3. But don't fail if we're running from a source checkout, as we don't want to anger Mercurial developers hacking on Python 3 nor Mercurial's test automation running from source checkouts. People running setup.py from source checkouts could still fall through a Python 3 crack. But at least the `pip install Mercurial` attempt will get nipped in the bud.

help: fix wording describing SSH requirements

graphlog: document what "_" and "*" mean Documenting "*" should've been a part of 9b3f95d9783d, but I somehow didn't notice that the symbols are explained in the command's help text.