drop old git_require checks

We've been requiring git-1.7.2 since Oct 2012, so we can safely drop
the individual checks sprinkled throughout the code base for older.

Change-Id: I1737fff7b3f27f475960b0bff9cb300aefd5d108
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/253135
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>