doc: fix the max number of branches shown by "show-branch"

The number to be displayed is calculated by the following defined in
object.h:

    #define REV_SHIFT        2
    #define MAX_REVS        (FLAG_BITS - REV_SHIFT)

FLAG_BITS is currently 28, so 26 is the correct number.

Signed-off-by: Rikita Ishikawa <lagrange.resolvent@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-show-branch.txt

@@ -22,7 +22,7 @@ Shows the commit ancestry graph starting from the commits named
 with <rev>s or <glob>s (or all refs under refs/heads
 and/or refs/tags) semi-visually.
 
-It cannot show more than 29 branches and commits at a time.
+It cannot show more than 26 branches and commits at a time.
 
 It uses `showbranch.default` multi-valued configuration items if
 no <rev> or <glob> is given on the command line.