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: NPaul Mackerras <paulus@samba.org>

gitk

@@ -7478,9 +7478,9 @@ set revtreeargs {}
 set cmdline_files {}
 set i 0
 foreach arg $argv {
-    switch -regexp -- $arg {
-	"^$" { }
-	"^-d" { set datemode 1 }
+    switch -- $arg {
+	"" { }
+	"-d" { set datemode 1 }
 	"--" {
 	    set cmdline_files [lrange $argv [expr {$i + 1}] end]
 	    break