git-gui: New Git version check support routine

git-gui: New Git version check support routine

Some newer features of git-gui want to rely on features that are
new to Git 1.5.3.  Since they were added as part of the 1.5.3
development series we cannot use those features with versions of
Git that are older than 1.5.3, such as from the stable 1.5.2 series.

We introduce [git-version >= 1.5.3] to allow the caller to get a
response of 0 if the current version of git is < 1.5.3 and 1 if
the current version of git is >= 1.5.3.  This makes it easy to
setup conditional code based upon the version of Git available to
us at runtime.

Instead of parsing the version text by hand we now use the Tcl
[package vcompare] subcommand to compare the two version strings.
This works nicely, as Tcl as already done all of the hard work
of doing version comparsions.  But we do have to remove the Git
specific components such as the Git commit SHA-1, commit count and
release candidate suffix (rc) as we want only the final release
version number.

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