whatchanged documentation: share description of --pretty with others

The documentation had its own description for --pretty and did not
include pretty-options/formats as documentation for other commands in
the "log" family did.

Signed-off-by: Denis Cheng <crquan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-whatchanged.txt

@@ -38,11 +38,6 @@ OPTIONS
 	Show git internal diff output, but for the whole tree,
 	not just the top level.
 
---pretty=<format>::
-	Controls the output format for the commit logs.
-	<format> can be one of 'raw', 'medium', 'short', 'full',
-	and 'oneline'.
-
 -m::
 	By default, differences for merge commits are not shown.
 	With this flag, show differences to that commit from all
@@ -51,6 +46,10 @@ OPTIONS
 However, it is not very useful in general, although it
 *is* useful on a file-by-file basis.
 
+include::pretty-options.txt[]
+
+include::pretty-formats.txt[]
+
 Examples
 --------
 git-whatchanged -p v2.6.12.. include/scsi drivers/scsi::