gitk: Fix display of newly-created tags

If the user creates a tag with the "create tag" dialog in gitk and
then clicks on the newly-created tag, its contents don't get
displayed.  The reason is that rereadrefs hasn't been called, meaning
the tag doesn't exist in $tagobjid.  This causes the cat-file to fail.
Instead of using $tagobjid, pass the $tag directly, ensuring the tag
contents are populated correctly.

Signed-off-by: David Dulson <dave@dulson.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>

gitk

@@ -10493,7 +10493,7 @@ proc showtag {tag isnew} {
     set linknum 0
     if {![info exists tagcontents($tag)]} {
 	catch {
-	    set tagcontents($tag) [exec git cat-file tag $tagobjid($tag)]
+           set tagcontents($tag) [exec git cat-file tag $tag]
 	}
     }
     if {[info exists tagcontents($tag)]} {