rev-list: add --count to usage guide

--count should be mentioned in the usage guide, this updates code and
documentation.

Signed-off-by: Lawrence Siebert <lawrencesiebert@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-rev-list.txt

@@ -56,6 +56,7 @@ SYNOPSIS
 	     [ --reverse ]
 	     [ --walk-reflogs ]
 	     [ --no-walk ] [ --do-walk ]
+	     [ --count ]
 	     [ --use-bitmap-index ]
 	     <commit>... [ \-- <paths>... ]
 

builtin/rev-list.c

@@ -42,6 +42,7 @@ static const char rev_list_usage[] =
 "    --abbrev=<n> | --no-abbrev\n"
 "    --abbrev-commit\n"
 "    --left-right\n"
+"    --count\n"
 "  special purpose:\n"
 "    --bisect\n"
 "    --bisect-vars\n"