bpo-27593: Revise git SCM build info. (#744)

Use --short form of git hash.  Use output from "git describe" for tag.

Expected outputs:
1. previous hg
2. previous git
3. updated git

Release (tagged) build:
1. Python 3.7.0a0 (v3.7.0a0:4def2a2901a5, ...
2. Python 3.7.0a0 (v3.7.0a0^0:05f53735c8912f8df1077e897f052571e13c3496, ...
3. Python 3.7.0a0 (v3.7.0a0:05f53735c8, ...

Development build:
1. Python 3.7.0a0 (default:41df79263a11, ...
2. Python 3.7.0a0 (master:05f53735c8912f8df1077e897f052571e13c3496, ...
3. Python 3.7.0a0 (heads/master-dirty:05f53735c8, ...

"dirty" means the working tree has uncommitted changes.
See "git help describe" for more info.