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>

git_command.py

@@ -28,8 +28,17 @@ from repo_trace import REPO_TRACE, IsTrace, Trace
 from wrapper import Wrapper
 
 GIT = 'git'
-# Should keep in sync with the "repo" launcher file.
-MIN_GIT_VERSION = (2, 10, 2)
+# NB: These do not need to be kept in sync with the repo launcher script.
+# These may be much newer as it allows the repo launcher to roll between
+# different repo releases while source versions might require a newer git.
+#
+# The soft version is when we start warning users that the version is old and
+# we'll be dropping support for it.  We'll refuse to work with versions older
+# than the hard version.
+#
+# git-1.7 is in (EOL) Ubuntu Precise.  git-1.9 is in Ubuntu Trusty.
+MIN_GIT_VERSION_SOFT = (1, 9, 1)
+MIN_GIT_VERSION_HARD = (1, 7, 2)
 GIT_DIR = 'GIT_DIR'
 
 LAST_GITDIR = None