gitk(1): mention --all

Signed-off-by: Jonas Fonseca <fonseca@diku.dk>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/gitk.txt

@@ -37,6 +37,10 @@ frequently used options.
 
 	Show commits older than a specific date.
 
+--all::
+
+	Show all branches.
+
 <revs>::
 
 	Limit the revisions to show. This can be either a single revision
@@ -63,6 +67,11 @@ gitk --since="2 weeks ago" \-- gitk::
 	The "--" is necessary to avoid confusion with the *branch* named
 	'gitk'
 
+gitk --max-count=100 --all -- Makefile::
+
+	Show at most 100 changes made to the file 'Makefile'. Instead of only
+	looking for changes in the current branch look in all branches.
+
 See Also
 --------
 'qgit(1)'::