gitk: Fix bug introduced by previous commit

gitk: Fix bug introduced by previous commit

When I added the "--" case to the code scanning the arguments, I missed
the fact that since the switch statement uses -regexp, the "--" case
will match any argument containing "--", e.g. "--all".  This fixes it
by taking out the -regexp (since we don't actually need regular
expression matching) and adjusting the match strings.

A side effect of this is that previously any argument starting with
"-d" would be taken to indicate date mode; now the argument has to be
exactly "-d" if you want date mode.

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