gitk: Take only numeric version components when computing $git_version

gitk: Take only numeric version components when computing $git_version

This fixes errors running with release candidate versions of Git:
  Error in startup script: expected version number but got "1.7.4-rc0"

Also, $git_version is no longer artificially limited to three
components.  That limitation was added by commit 194bbf6cc8c2
("gitk: Handle msysGit version during version comparisons") to deal
with msysGit version strings like “1.6.4.msysgit.0”, and we don’t need
it now.  Hence as another side effect, this enables showing notes with
git version 1.6.6.2 or 1.6.6.3, as originally intended by commit
7defefb13427 ("gitk: Show notes by default (like git log does").

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
Reported-by: Mathias Lafeldt <misfire@debugon.org>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>