git-gui: Basic version check to ensure git 1.5.0 or later is used.

git-gui: Basic version check to ensure git 1.5.0 or later is used.

This is a very crude (but hopefully effective) check against the
`git` executable found in our PATH.  Some of the subcommands and
options that git-gui requires to be present to operate were created
during the 1.5.0 development cycle, so 1.5 is the minimum version
of git that we can expect to support.

There actually are early releases of 1.5 (e.g. 1.5.0-rc0) that
don't have everything we expect (like `blame --incremental`) but
these are purely academic at this point.  1.5.0 final was tagged
and released just a few hours ago.  The release candidates will
(hopefully) fade into the dark quickly.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>