repo: lower min version of git a bit

We were perhaps a bit too hasty to jump to git-2.10.  Existing LTS
releases of Ubuntu are quite old still: Trusty has 1.9 while Xenial
has 2.5.  While we plan on dropping support for those eventually as
we migrate to Python 3.6, we don't need to be so strict just yet on
the git versions.

We also want to disconnect the version the repo launcher requires
from the version the rest of the source tree requires.  The repo
launcher doesn't need as many features, and being flexible there
allows us more freedom to upgrade & rollback as needed.

So we'll allow git-1.7 again, but start warning on any users older
than git-1.9.  This aligns better with existing LTS releases, and
gives users a chance to start upgrading before we cut them off.

Change-Id: I140305dd8e42c9719c84e2aee0dc6a5c5b18da25
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/254573


Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>