contrib/subtree: ignore log.date configuration

git-subtree's log format string uses "%ad" and "%cd", which
respect the user's configured log.date value.

This is problematic for git-subtree because it needs to use real
dates so that copied commits come through unchanged.

Add a test and tweak the format strings to use %aD and %cD
so that the default date format is used instead.

Reported-by: Bryan Jacobs <b@q3q.us>
Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/subtree/git-subtree.sh

@@ -298,7 +298,7 @@ copy_commit()
 	# We're going to set some environment vars here, so
 	# do it in a subshell to get rid of them safely later
 	debug copy_commit "{$1}" "{$2}" "{$3}"
-	git log -1 --pretty=format:'%an%n%ae%n%ad%n%cn%n%ce%n%cd%n%B' "$1" |
+	git log -1 --pretty=format:'%an%n%ae%n%aD%n%cn%n%ce%n%cD%n%B' "$1" |
 	(
 		read GIT_AUTHOR_NAME
 		read GIT_AUTHOR_EMAIL

contrib/subtree/t/t7900-subtree.sh

@@ -94,6 +94,10 @@ test_expect_success 'add sub3' '
 # Back to mainline
 cd ..
 
+test_expect_success 'enable log.date=relative to catch errors' '
+	git config log.date relative
+'
+
 test_expect_success 'add main4' '
         create main4 &&
         git commit -m "main4" &&