shortlog: test and document --committer option

This puts the final touches on the feature added by
fbfda15fb8 (shortlog: group by committer information,
2016-10-11).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-shortlog.txt

@@ -47,6 +47,10 @@ OPTIONS
 
 	Each pretty-printed commit will be rewrapped before it is shown.
 
+-c::
+--committer::
+	Collect and show committer identities instead of authors.
+
 -w[<width>[,<indent1>[,<indent2>]]]::
 	Linewrap the output by wrapping each line at `width`.  The first
 	line of each entry is indented by `indent1` spaces, and the second

t/t4201-shortlog.sh

@@ -190,4 +190,17 @@ test_expect_success 'shortlog with --output=<file>' '
 	test_line_count = 3 shortlog
 '
 
+test_expect_success 'shortlog --committer (internal)' '
+	cat >expect <<-\EOF &&
+	     3	C O Mitter
+	EOF
+	git shortlog -nsc HEAD >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'shortlog --committer (external)' '
+	git log --format=full | git shortlog -nsc >actual &&
+	test_cmp expect actual
+'
+
 test_done