gitk: Fix display of newly-created tags

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 <redacted>
Signed-off-by: Paul Mackerras <redacted>