gitk: Improve cherry-pick error handling
This adds to the regexps that are used to work out what sort of error
we encountered in trying to do a cherry-pick so that it recognizes
some additional common error messages. It adds a confirmation dialog
when the error is a merge conflict so the user can choose whether or
not to run git citool. Finally, it arranges to update the display
after a cherry-pick failed so that any local changes made by the
cherry-pick become visible.
Signed-off-by: Paul Mackerras <paulus@samba.org>