diff --abbrev: document --abbrev=<n> form.

It was implemented there but was not advertised.

Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/diff-options.txt

@@ -18,12 +18,13 @@
 	object name of pre- and post-image blob on the "index"
 	line when generating a patch format output.	
 
---abbrev::
+--abbrev[=<n>]::
 	Instead of showing the full 40-byte hexadecimal object
 	name in diff-raw format output and diff-tree header
-	lines, show only handful prefix.  This is independent of
-	--full-index option above, which controls the diff-patch
-	output format.
+	lines, show only handful dhexigits prefix.  This is
+	independent of --full-index option above, which controls
+	the diff-patch output format.  Non default number of
+	digits can be specified with --abbrev=<n>.
 
 -B::
 	Break complete rewrite changes into pairs of delete and create.