git: raise hard version to 1.9.1

Debian 7 Wheezy went EOL in May 2018.  We don't need to carry support
for that anymore as there have been 5 major releases since.  Ubuntu
Precise went EOL in Apr 2019 (including the extended support phase).
That means we can bump the required git version from 1.7.9 to 1.9.1.

git-1.7.9 was released in 2012 while git-1.9.1 was released in 2014.
So that shouldn't be a problem either.  And we've been warning people
using git versions older than 1.9.1 for 3 years now that they need to
upgrade.

Change-Id: Ifbbf72f51010b0a944c2785895d1b605333f9146
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/415637


Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Tested-by: Mike Frysinger <vapier@google.com>
Commit-Queue: Mike Frysinger <vapier@google.com>