Document the --histogram diff option

Signed-off-by: NThomas Rast <trast@student.ethz.ch>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/diff-options.txt

@@ -52,6 +52,9 @@ endif::git-format-patch[]
 --patience::
 	Generate a diff using the "patience diff" algorithm.
 
+--histogram::
+	Generate a diff using the "histogram diff" algorithm.
+
 --stat[=<width>[,<name-width>[,<count>]]]::
 	Generate a diffstat.  You can override the default
 	output width for 80-column terminal by `--stat=<width>`.