gitk: Display the date of a tag in a human-friendly way

By selecting a tag within gitk you can display information about it.
This information is output by using the command

 'git cat-file tag <tagid>'

This outputs the *raw* information from the tag, amongst which is the
time - in seconds since the epoch. As useful as that value is, I find it
a lot easier to read and process time which it is something like:

 "Mon Dec 31 14:26:11 2012 -0800"

This change will modify the display of tags in gitk like so:

  @@ -1,7 +1,7 @@
   object 5d417842ef
   type commit
   tag v1.8.1
  -tagger Junio C Hamano <gitster@pobox.com> 1356992771 -0800
  +tagger Junio C Hamano <gitster@pobox.com> Mon Dec 31 14:26:11 2012 -0800

   Git 1.8.1
   -----BEGIN PGP SIGNATURE-----

Signed-off-by: Anand Kumria <wildfire@progsoc.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>

gitk

@@ -10831,7 +10831,7 @@ proc showtag {tag isnew} {
     set linknum 0
     if {![info exists cached_tagcontent($tag)]} {
 	catch {
-           set cached_tagcontent($tag) [exec git cat-file tag $tag]
+           set cached_tagcontent($tag) [exec git cat-file -p $tag]
 	}
     }
     if {[info exists cached_tagcontent($tag)]} {