git-gui: generic version trimming

git-gui: generic version trimming

Rather than attempting to trim off all the non-version number suffixes
from the 'git version' result, let us scan along from the beginning until
we find a non-numeric part and stop there. Any such dot-version number will
be compatible with the Tcl package version comparison command which is the
aim of this code.

Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>