testlib: add a small scrip to help process to synchronise using file Creating and waiting for files is a robust way to synchronise two processes running concurrently. We already use this approach in various tests. I am adding a official script to do so before adding more usage of this. Differential Revision: https://phab.mercurial-scm.org/D8189

setup: work around old versions of distutils breaking setup.py I'm not really sure how to trigger this, but we saw it in our build environment for Windows at Google. This fixed it. Sigh.

chgserver: update the umask cache before each run posix.py uses a global variable to store the umask value resulting in caching of it when using chg. We need to update it before each command run as the umask can change between commands. This fixes test-inherit-mode.t with chg.

tests: handle new error string from FreeBSD for dns entry not resolving Differential Revision: https://phab.mercurial-scm.org/D8333

notify: optional mail threading based on obsmarker When notify.reply is set and a changeset has a predecessor in the repository, include In-Reply-To pointing to the message-id that would have been generated for the oldest predecessor. This allows mail threading like Phabricator for common cases like rebasing changes, but will be optimal for cases like folding. Differential Revision: https://phab.mercurial-scm.org/D8172

pathutil: document that dirs map type implies manifest/dirstate processing

git: pass a list to pathutil.dirs to indicate that it is a manifest The python implementation of pathutil.dirs just uses a for loop which happens to work the same on both dicts and lists. The rust implementation actually figures out which of the two types it is, and directs the execution to either dirstate or manifest processing.

git: implement basic bookmark activation This is very limited, but it allows 'hg update foo' when already on foo. The caching is based on bmstore's caching.

git: implement a basic checkconflict bookmark store method It is heavily based on bmstore's own checkconflict.

git: abort when attempting to set a branch Given the mapping we use (namely, a git head is a bookmark), it is better to error out with a hint.

git: remove obsolete todo item The changes in 02c47b74366c cleaned up the requirement check.

tests: don't run test-update-atomic.t on chg I am unable to find a good way to make `showwrites` extension in it to work with chg. Also putting the use of showwrites inside `if no-chg` will defeat the purpose of test, so I just made the test no-op on chg.