tg.sh: require Git 1.9.2 or later

tg.sh: require Git 1.9.2 or later

TopGit has been using the `git update-ref --stdin` mechanism since
topgit-0.19.7 including the `create` command of that mechanism.

Unfortunately the `create` command does not work properly until Git
version 1.9.2 (see Git commit b9d56b5dd9a8819c "update-ref: fail
create operation over stdin if ref already exists").

Update the minimum required Git version from 1.9.0 to 1.9.2 to
reflect this.

Git 1.9.0 was released on 2014-02-14 and Git 1.9.2 on 2014-04-09.

The effect of this change is to require a Git that is almost 2
months more recent than previously required, but still over 4 years
older than this commit itself.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>