git-gui: accept versions containing text annotations, like 1.5.3.mingw.1

git-gui: accept versions containing text annotations, like 1.5.3.mingw.1

This commit teaches git-gui to accept versions with annotations
that start with text and optionally end with a dot followed by
a number.

This is needed by the current versioning scheme of msysgit,
which uses versions like 1.5.3.mingw.1. However, the changes
is not limited to this use case. Any version of the form
<numeric version>.<anytext>.<number> would be parsed and only
the starting <numeric version> used for validation.

[sp: Minor edit to remove unnecessary group matching]

Signed-off-by: Steffen Prohaska <prohaska@zib.de>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>