merge-recursive: fix assumption that head tree being merged is HEAD

`git merge-recursive` does a three-way merge between user-specified trees
base, head, and remote.  Since the user is allowed to specify head, we can
not necesarily assume that head == HEAD.

Modify index_has_changes() to take an extra argument specifying the tree
to compare against.  If NULL, it will compare to HEAD.  We then use this
from merge-recursive to make sure we compare to the user-specified head.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/am.c

@@ -1763,7 +1763,7 @@ static void am_run(struct am_state *state, int resume)
 
 	refresh_and_write_cache();
 
-	if (index_has_changes(&the_index, &sb)) {
+	if (index_has_changes(&the_index, NULL, &sb)) {
 		write_state_bool(state, "dirtyindex", 1);
 		die(_("Dirty index: cannot apply patches (dirty: %s)"), sb.buf);
 	}
@@ -1820,7 +1820,8 @@ static void am_run(struct am_state *state, int resume)
 			 * Applying the patch to an earlier tree and merging
 			 * the result may have produced the same tree as ours.
 			 */
-			if (!apply_status && !index_has_changes(&the_index, NULL)) {
+			if (!apply_status &&
+			    !index_has_changes(&the_index, NULL, NULL)) {
 				say(state, stdout, _("No changes -- Patch already applied."));
 				goto next;
 			}
@@ -1878,7 +1879,7 @@ static void am_resolve(struct am_state *state)
 
 	say(state, stdout, _("Applying: %.*s"), linelen(state->msg), state->msg);
 
-	if (!index_has_changes(&the_index, NULL)) {
+	if (!index_has_changes(&the_index, NULL, NULL)) {
 		printf_ln(_("No changes - did you forget to use 'git add'?\n"
 			"If there is nothing left to stage, chances are that something else\n"
 			"already introduced the same changes; you might want to skip this patch."));

cache.h

@@ -218,6 +218,7 @@ struct cache_entry {
 /* Forward structure decls */
 struct pathspec;
 struct child_process;
+struct tree;
 
 /*
  * Copy the sha1 and stat state of a cache entry from one to
@@ -627,12 +628,14 @@ extern void move_index_extensions(struct index_state *dst, struct index_state *s
 extern int unmerged_index(const struct index_state *);
 
 /**
- * Returns 1 if istate differs from HEAD, 0 otherwise.  When on an unborn
+ * Returns 1 if istate differs from tree, 0 otherwise.  If tree is NULL,
- * branch, returns 1 if there are entries in istate, 0 otherwise.  If an
+ * compares istate to HEAD.  If tree is NULL and on an unborn branch,
- * strbuf is provided, the space-separated list of files that differ will
+ * returns 1 if there are entries in istate, 0 otherwise.  If an strbuf is
- * be appended to it.
+ * provided, the space-separated list of files that differ will be appended
+ * to it.
  */
 extern int index_has_changes(const struct index_state *istate,
+			     struct tree *tree,
 			     struct strbuf *sb);
 
 extern int verify_path(const char *path, unsigned mode);

merge-recursive.c

@@ -1983,7 +1983,7 @@ int merge_trees(struct merge_options *o,
 	if (oid_eq(&common->object.oid, &merge->object.oid)) {
 		struct strbuf sb = STRBUF_INIT;
 
-		if (!o->call_depth && index_has_changes(&the_index, &sb)) {
+		if (!o->call_depth && index_has_changes(&the_index, head, &sb)) {
 			err(o, _("Your local changes to the following files would be overwritten by merge:\n  %s"),
 			    sb.buf);
 			return -1;

read-cache.c

@@ -1986,22 +1986,26 @@ int unmerged_index(const struct index_state *istate)
 	return 0;
 }
 
-int index_has_changes(const struct index_state *istate, struct strbuf *sb)
+int index_has_changes(const struct index_state *istate,
+		      struct tree *tree,
+		      struct strbuf *sb)
 {
-	struct object_id head;
+	struct object_id cmp;
 	int i;
 
 	if (istate != &the_index) {
 		BUG("index_has_changes cannot yet accept istate != &the_index; do_diff_cache needs updating first.");
 	}
-	if (!get_oid_tree("HEAD", &head)) {
+	if (tree)
+		cmp = tree->object.oid;
+	if (tree || !get_oid_tree("HEAD", &cmp)) {
 		struct diff_options opt;
 
 		diff_setup(&opt);
 		opt.flags.exit_with_status = 1;
 		if (!sb)
 			opt.flags.quick = 1;
-		do_diff_cache(&head, &opt);
+		do_diff_cache(&cmp, &opt);
 		diffcore_std(&opt);
 		for (i = 0; sb && i < diff_queued_diff.nr; i++) {
 			if (i)

t/t6044-merge-unrelated-index-changes.sh

@@ -126,7 +126,7 @@ test_expect_success 'recursive, when merge branch matches merge base' '
 	test_path_is_missing .git/MERGE_HEAD
 '
 
-test_expect_failure 'merge-recursive, when index==head but head!=HEAD' '
+test_expect_success 'merge-recursive, when index==head but head!=HEAD' '
 	git reset --hard &&
 	git checkout C^0 &&