gitk: Fix handling of flag arguments

gitk: Fix handling of flag arguments

Despite the name, the --revs-only flag to git rev-parse doesn't make
it output only revision IDs.  It makes it output only arguments that
are suitable for giving to git rev-list.  So make start_rev_list and
updatecommits cope with arguments output by git rev-parse that aren't
revision IDs.  This way we won't get an error when an argument such as
"-300" has been given to gitk and the view is updated.

Signed-off-by: Paul Mackerras <paulus@samba.org>