gitk: Improve handling of -- and ambiguous arguments

gitk: Improve handling of -- and ambiguous arguments

This makes gitk more consistent with git rev-list and git log in its
handling of arguments that could be either a revision or a filename;
now gitk displays an error message and quits, rather than treating it
as a revision and getting an error in the underlying git log.  Now
gitk always passes "--" to git log even if no filenames are being
specified.

It also makes gitk display errors in invoking git log in a window
rather than on stderr, and makes gitk stop looking for a -d flag
when it sees a "--" argument.

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