[3.5] bpo-27593: Get SCM build info from git instead of hg. (#446) (#454) (#455)

[3.5] bpo-27593: Get SCM build info from git instead of hg. (#446) (#454) (#455)

* bpo-27593: Get SCM build info from git instead of hg. (#446)

sys.version and the platform module python_build(),
python_branch(), and python_revision() functions now use
git information rather than hg when building from a repo.

Based on original patches by Brett Cannon and Steve Dower.
(cherry picked from commit 5c4b0d063aba0a68c325073f5f312a2c9f40d178)

(cherry picked from commit 95c50e5aed9e5683676e18349dd94b11901a66b3)