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.
diff --git a/Include/pythonrun.h b/Include/pythonrun.h
index cfb02b0..f0f4e38 100644
--- a/Include/pythonrun.h
+++ b/Include/pythonrun.h
@@ -111,11 +111,10 @@
 PyAPI_FUNC(const char *) Py_GetCopyright(void);
 PyAPI_FUNC(const char *) Py_GetCompiler(void);
 PyAPI_FUNC(const char *) Py_GetBuildInfo(void);
-PyAPI_FUNC(const char *) _Py_svnversion(void);
 PyAPI_FUNC(const char *) Py_SubversionRevision(void);
 PyAPI_FUNC(const char *) Py_SubversionShortBranch(void);
-PyAPI_FUNC(const char *) _Py_hgidentifier(void);
-PyAPI_FUNC(const char *) _Py_hgversion(void);
+PyAPI_FUNC(const char *) _Py_gitidentifier(void);
+PyAPI_FUNC(const char *) _Py_gitversion(void);
 
 /* Internal -- various one-time initializations */
 PyAPI_FUNC(PyObject *) _PyBuiltin_Init(void);