Merge branch 'da/subtree-date-confusion' into maint

"git subtree" (in contrib/) depended on "git log" output to be
stable, which was a no-no.  Apply a workaround to force a
particular date format.

* da/subtree-date-confusion:
  contrib/subtree: ignore log.date configuration

Author: gitster

contrib/subtree/git-subtree.sh

@@ -305,7 +305,7 @@ 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" &&