git: raise hard version to 1.7.9

Debian 6 Squeeze went EOL in Feb 2016.  We don't need to carry support
for that anymore as there have been 6 major releases since.  That means
we can bump the required git version from 1.7.2 to 1.7.9.  Ubuntu Precise
shipped with the latter.

git-1.7.2 was released in 2010 while git-1.7.9 was released in 2012.
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: I7712f110ea158297b489b8379b112c6700b21a46
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/415097


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