bpo-27593: Get SCM build info from git instead of hg (#1327)

bpo-27593: Get SCM build info from git instead of hg (#1327)

Based on commit 5c4b0d063aba0a68c325073f5f312a2c9f40d178 by Ned
Deily, which is based on original patches by Brett Cannon and Steve
Dower.

Remove also the private _Py_svnversion() function and SVNVERSION
variable.

Note: Py_SubversionRevision() and Py_SubversionShortBranch() are
unchanged, they are part of the public API.