Move MERGE_RR from .git/rr-cache/ into .git/

If you want to reuse the rerere cache in another repository, and set
a symbolic link to it, you do not want to have the two repositories
interfer with each other by accessing the _same_ MERGE_RR.

For example, if you use contrib/git-new-workdir to set up a second
working directory, and you have a conflict in one working directory,
but commit in the other working directory first, the wrong "resolution"
will be recorded.

The easy solution is to move MERGE_RR out of the rr-cache/ directory,
which also corresponds with the notion that rr-cache/ contains cached
resolutions, not some intermediate temporary states.

Noticed by Kalle Olavi Niemitalo.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

branch.c

@@ -166,7 +166,7 @@ void create_branch(const char *head,
 void remove_branch_state(void)
 {
 	unlink(git_path("MERGE_HEAD"));
-	unlink(git_path("rr-cache/MERGE_RR"));
+	unlink(git_path("MERGE_RR"));
 	unlink(git_path("MERGE_MSG"));
 	unlink(git_path("SQUASH_MSG"));
 }

builtin-rerere.c

@@ -429,7 +429,7 @@ static int setup_rerere(struct path_list *merge_rr)
 	if (!is_rerere_enabled())
 		return -1;
 
-	merge_rr_path = xstrdup(git_path("rr-cache/MERGE_RR"));
+	merge_rr_path = xstrdup(git_path("MERGE_RR"));
 	fd = hold_lock_file_for_update(&write_lock, merge_rr_path, 1);
 	read_rr(merge_rr);
 	return fd;

t/t4200-rerere.sh

@@ -57,7 +57,7 @@ test_expect_success 'conflicting merge' '
 	! git merge first
 '
 
-sha1=$(sed -e 's/	.*//' .git/rr-cache/MERGE_RR)
+sha1=$(sed -e 's/	.*//' .git/MERGE_RR)
 rr=.git/rr-cache/$sha1
 test_expect_success 'recorded preimage' "grep ^=======$ $rr/preimage"
 
@@ -143,7 +143,7 @@ test_expect_success 'rerere kicked in' "! grep ^=======$ a1"
 test_expect_success 'rerere prefers first change' 'test_cmp a1 expect'
 
 rm $rr/postimage
-echo "$sha1	a1" | perl -pe 'y/\012/\000/' > .git/rr-cache/MERGE_RR
+echo "$sha1	a1" | perl -pe 'y/\012/\000/' > .git/MERGE_RR
 
 test_expect_success 'rerere clear' 'git rerere clear'
 
@@ -190,7 +190,7 @@ test_expect_success 'file2 added differently in two branches' '
 	git add file2 &&
 	git commit -m version2 &&
 	! git merge fourth &&
-	sha1=$(sed -e "s/	.*//" .git/rr-cache/MERGE_RR) &&
+	sha1=$(sed -e "s/	.*//" .git/MERGE_RR) &&
 	rr=.git/rr-cache/$sha1 &&
 	echo Cello > file2 &&
 	git add file2 &&