git-gui: Let the user continue even if we cannot understand git version

git-gui: Let the user continue even if we cannot understand git version

Some users may do odd things, like tag their own private version of
Git with an annotated tag such as 'testver', then compile that git
and try to use it with git-gui.  In such a case `git --version` will
give us 'git version testver', which is not a numeric argument that
we can pass off to our version comparsion routine.

We now check that the cleaned up git version is a going to pass the
version comparsion routine without failure.  If it has a non-numeric
component, or lacks at least a minor revision then we ask the user to
confirm they really want to use this version of git within git-gui.
If they do we shall assume it is git 1.5.0 and run with only the code
that will support.

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