templates: detect messages that contain a separator line

Messages that contain "---" separator lines will be truncated by
"git am". This often surprises users so add a check to the sample
"commit-msg" hook to reject such messages. As it's conceivable that
someone is using "---" as their comment string we delete any commented
lines before checking for a separator. The trailing ".*" when matching
commented lines ensures that if the comment string ends with a "$"
it is not treated as an anchor.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: phillipwood

templates/hooks/commit-msg.sample

@@ -39,9 +39,16 @@ comment_re="$(
 		}'
 )"
 line="$(sed -n -e "/^${comment_re} -\{8,\} >8 -\{8,\}\$/q
+		   /^${comment_re}.*/d
+		   /^---\$/{p;q;}
 		   /^diff -/{p;q;}
 		   /^Index: /{p;q;}" "$1")"
-if test -n "$line"
+if test "$line" = "---"
+then
+	echo >&2 "Message contains a '---' separator line that will confuse"
+	echo >&2 "'git am'. To fix this indent the '---' line."
+	ret=1
+elif test -n "$line"
 then
 	echo >&2 "Message contains a diff that will confuse 'git am'."
 	echo >&2 "To fix this indent the diff."