Use git commit id as release identifier in development builds

Use git commit id as release identifier in development builds

This change adds support for using latest git commit id as release name
and id. This is done only if:

1) No existing release info file is detected. Release script will
generate that file so actual release builds won't accidentally use
development identifiers.

2) .git dir is detected.

3) Python binary is found, so this change doesn't add mandatory
build-time dependency to python.

Bug: 18445909
Change-Id: I3253b8b05f14e507774bb43331872607ffe6c1d0