phabricator: check associated Differential Revision from commit message Previously, only tags can "associate" a changeset to a Differential Revision. But the usual pattern (arc patch or hg phabread) is to put the Differential Revision URL in commit message. This patch makes the code read commit message to find associated Differential Revision if associated tags are not found. This makes some workflows possible. For example, if the author loses their repo, or switch to another computer, they can continue download their own patches from Phabricator and update them without needing to manually create tags.

tests: replace yet more calls to `python` with $PYTHON These are some simple cases. More to come in a future change. Reviewers: krbullock Reviewed By: krbullock Differential Revision: https://phab.mercurial-scm.org/D4

tests: capitalize Python when it's not used as a command name This avoids some false positives in an upcoming check-code rule. Reviewers: krbullock Reviewed By: krbullock Differential Revision: https://phab.mercurial-scm.org/D3

merge with stable