templates: detect commit messages containing diffs

If a commit message contains a diff that is not indented then "git
am" will treat that diff as part of the patch rather than as part
of the commit message. This allows it to apply email messages that
were created by adding a commit message in front of a regular diff
without adding the "---" separator used by "git format-patch". This
often surprises users [1-4] so add a check to the sample "commit-msg"
hook to reject messages that would confuse "git am".

Detecting if the message contains a diff is complicated by the hook
being passed the message before it is cleaned up so we need to ignore
any diffs below the scissors line. There are also two possible
config keys to check to find the comment character at the start
of the scissors line.

[1] https://lore.kernel.org/git/bcqvh7ahjjgzpgxwnr4kh3hfkksfruf54refyry3ha7qk7dldf@fij5calmscvm
[2] https://lore.kernel.org/git/ca13705ae4817ffba16f97530637411b59c9eb19.camel@scientia.org/
[3] https://lore.kernel.org/git/d0b577825124ac684ab304d3a1395f3d2d0708e8.1662333027.git.matheus.bernardino@usp.br/
[4] https://lore.kernel.org/git/CAFOYHZC6Qd9wkoWPcTJDxAs9u=FGpHQTkjE-guhwkya0DRVA6g@mail.gmail.com/

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

@@ -15,10 +15,37 @@
 # SOB=$(git var GIT_AUTHOR_IDENT | sed -n 's/^\(.*>\).*$/Signed-off-by: \1/p')
 # grep -qs "^$SOB" "$1" || echo "$SOB" >> "$1"
 
-# This example catches duplicate Signed-off-by lines.
+# This example catches duplicate Signed-off-by lines and messages that
+# would confuse 'git am'.
+
+ret=0
 
 test "" = "$(grep '^Signed-off-by: ' "$1" |
 	 sort | uniq -c | sed -e '/^[ 	]*1[ 	]/d')" || {
 	echo >&2 Duplicate Signed-off-by lines.
-	exit 1
+	ret=1
 }
+
+comment_re="$(
+	{
+		git config --get-regexp "^core\.comment(char|string)\$" ||
+			echo '#'
+	} | sed -n -e '
+		${
+			s/^[^ ]* //
+			s|[][*./\]|\\&|g
+			s/^auto$/[#;@!$%^&|:]/
+			p
+		}'
+)"
+line="$(sed -n -e "/^${comment_re} -\{8,\} >8 -\{8,\}\$/q
+		   /^diff -/{p;q;}
+		   /^Index: /{p;q;}" "$1")"
+if test -n "$line"
+then
+	echo >&2 "Message contains a diff that will confuse 'git am'."
+	echo >&2 "To fix this indent the diff."
+	ret=1
+fi
+
+exit $ret