gitk: Improve cherry-pick error handling

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>