checkout: respect GIT_REFLOG_ACTION

GIT_REFLOG_ACTION is an environment variable specifying the reflog
message to write after an action is completed.  Several other commands
including merge, reset, and commit respect it.

Fix the failing tests in t/checkout-last by making checkout respect it
too.  You can now expect

  $ git checkout -

to work as expected after any operation that internally uses "checkout"
as its implementation detail, e.g. "rebase".

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/checkout.c

@@ -587,7 +587,7 @@ static void update_refs_for_switch(const struct checkout_opts *opts,
 				   struct branch_info *new)
 {
 	struct strbuf msg = STRBUF_INIT;
-	const char *old_desc;
+	const char *old_desc, *reflog_msg;
 	if (opts->new_branch) {
 		if (opts->new_orphan_branch) {
 			if (opts->new_branch_log && !log_all_ref_updates) {
@@ -620,8 +620,13 @@ static void update_refs_for_switch(const struct checkout_opts *opts,
 	old_desc = old->name;
 	if (!old_desc && old->commit)
 		old_desc = sha1_to_hex(old->commit->object.sha1);
-	strbuf_addf(&msg, "checkout: moving from %s to %s",
-		    old_desc ? old_desc : "(invalid)", new->name);
+
+	reflog_msg = getenv("GIT_REFLOG_ACTION");
+	if (!reflog_msg)
+		strbuf_addf(&msg, "checkout: moving from %s to %s",
+			old_desc ? old_desc : "(invalid)", new->name);
+	else
+		strbuf_insert(&msg, 0, reflog_msg, strlen(reflog_msg));
 
 	if (!strcmp(new->name, "HEAD") && !new->path && !opts->force_detach) {
 		/* Nothing to do. */

t/t2012-checkout-last.sh

@@ -116,15 +116,15 @@ test_expect_success 'master...' '
 	test "z$(git rev-parse --verify HEAD)" = "z$(git rev-parse --verify master^)"
 '
 
-test_expect_failure '"checkout -" works after a rebase A' '
+test_expect_success '"checkout -" works after a rebase A' '
 	git checkout master &&
 	git checkout other &&
 	git rebase master &&
 	git checkout - &&
 	test "z$(git symbolic-ref HEAD)" = "zrefs/heads/master"
 '
 
-test_expect_failure '"checkout -" works after a rebase A B' '
+test_expect_success '"checkout -" works after a rebase A B' '
 	git branch moodle master~1 &&
 	git checkout master &&
 	git checkout other &&
@@ -133,15 +133,15 @@ test_expect_failure '"checkout -" works after a rebase A B' '
 	test "z$(git symbolic-ref HEAD)" = "zrefs/heads/master"
 '
 
-test_expect_failure '"checkout -" works after a rebase -i A' '
+test_expect_success '"checkout -" works after a rebase -i A' '
 	git checkout master &&
 	git checkout other &&
 	git rebase -i master &&
 	git checkout - &&
 	test "z$(git symbolic-ref HEAD)" = "zrefs/heads/master"
 '
 
-test_expect_failure '"checkout -" works after a rebase -i A B' '
+test_expect_success '"checkout -" works after a rebase -i A B' '
 	git branch foodle master~1 &&
 	git checkout master &&
 	git checkout other &&