merge-recursive: exit early if index != head

We had a rule to enforce that the index matches head, but it was found
at the beginning of merge_trees() and would only trigger when
opt->call_depth was 0.  Since merge_recursive() doesn't call
merge_trees() until after returning from recursing, this meant that the
check wasn't triggered by merge_recursive() until it had first finished
all the intermediate merges to create virtual merge bases.  That is a
potentially huge amount of computation (and writing of intermediate
merge results into the .git/objects directory) before it errors out and
says, in effect, "Sorry, I can't do any merging because you have some
local changes that would be overwritten."

Further, not enforcing this requirement earlier allowed other bugs (such
as an unintentional unconditional dropping and reloading of the index in
merge_recursive() even when no recursion was necessary), to mask bugs in
other callers (which were fixed in the commit prior to this one).

Make sure we do the index == head check at the beginning of the merge,
and error out immediately if it fails.  While we're at it, fix a small
leak in the show-the-error codepath.

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

merge-recursive.c

@@ -3382,23 +3382,14 @@ static int process_entry(struct merge_options *opt,
 	return clean_merge;
 }
 
-int merge_trees(struct merge_options *opt,
-		struct tree *head,
-		struct tree *merge,
-		struct tree *common,
-		struct tree **result)
+static int merge_trees_internal(struct merge_options *opt,
+				struct tree *head,
+				struct tree *merge,
+				struct tree *common,
+				struct tree **result)
 {
 	struct index_state *istate = opt->repo->index;
 	int code, clean;
-	struct strbuf sb = STRBUF_INIT;
-
-	assert(opt->ancestor != NULL);
-
-	if (!opt->call_depth && repo_index_has_changes(opt->repo, head, &sb)) {
-		err(opt, _("Your local changes to the following files would be overwritten by merge:\n  %s"),
-		    sb.buf);
-		return -1;
-	}
 
 	if (opt->subtree_shift) {
 		merge = shift_tree_object(opt->repo, head, merge, opt->subtree_shift);
@@ -3502,11 +3493,11 @@ static struct commit_list *reverse_commit_list(struct commit_list *list)
  * Merge the commits h1 and h2, return the resulting virtual
  * commit object and a flag indicating the cleanness of the merge.
  */
-int merge_recursive(struct merge_options *opt,
-		    struct commit *h1,
-		    struct commit *h2,
-		    struct commit_list *ca,
-		    struct commit **result)
+static int merge_recursive_internal(struct merge_options *opt,
+				    struct commit *h1,
+				    struct commit *h2,
+				    struct commit_list *ca,
+				    struct commit **result)
 {
 	struct commit_list *iter;
 	struct commit *merged_common_ancestors;
@@ -3515,9 +3506,6 @@ int merge_recursive(struct merge_options *opt,
 	const char *ancestor_name;
 	struct strbuf merge_base_abbrev = STRBUF_INIT;
 
-	if (!opt->call_depth)
-		assert(opt->ancestor == NULL);
-
 	if (show(opt, 4)) {
 		output(opt, 4, _("Merging:"));
 		output_commit_title(opt, h1);
@@ -3571,7 +3559,7 @@ int merge_recursive(struct merge_options *opt,
 		saved_b2 = opt->branch2;
 		opt->branch1 = "Temporary merge branch 1";
 		opt->branch2 = "Temporary merge branch 2";
-		if (merge_recursive(opt, merged_common_ancestors, iter->item,
+		if (merge_recursive_internal(opt, merged_common_ancestors, iter->item,
 				    NULL, &merged_common_ancestors) < 0)
 			return -1;
 		opt->branch1 = saved_b1;
@@ -3587,12 +3575,12 @@ int merge_recursive(struct merge_options *opt,
 		repo_read_index(opt->repo);
 
 	opt->ancestor = ancestor_name;
-	clean = merge_trees(opt,
-			    repo_get_commit_tree(opt->repo, h1),
-			    repo_get_commit_tree(opt->repo, h2),
-			    repo_get_commit_tree(opt->repo,
-						 merged_common_ancestors),
-			    &mrtree);
+	clean = merge_trees_internal(opt,
+				     repo_get_commit_tree(opt->repo, h1),
+				     repo_get_commit_tree(opt->repo, h2),
+				     repo_get_commit_tree(opt->repo,
+							  merged_common_ancestors),
+				     &mrtree);
 	strbuf_release(&merge_base_abbrev);
 	if (clean < 0) {
 		flush_output(opt);
@@ -3613,6 +3601,61 @@ int merge_recursive(struct merge_options *opt,
 	return clean;
 }
 
+static int merge_start(struct merge_options *opt, struct tree *head)
+{
+	struct strbuf sb = STRBUF_INIT;
+
+	if (repo_index_has_changes(opt->repo, head, &sb)) {
+		err(opt, _("Your local changes to the following files would be overwritten by merge:\n  %s"),
+		    sb.buf);
+		strbuf_release(&sb);
+		return -1;
+	}
+
+	return 0;
+}
+
+static void merge_finalize(struct merge_options *opt)
+{
+	/* Common code for wrapping up merges will be added here later */
+}
+
+int merge_trees(struct merge_options *opt,
+		struct tree *head,
+		struct tree *merge,
+		struct tree *common,
+		struct tree **result)
+{
+	int clean;
+
+	assert(opt->ancestor != NULL);
+
+	if (merge_start(opt, head))
+		return -1;
+	clean = merge_trees_internal(opt, head, merge, common, result);
+	merge_finalize(opt);
+
+	return clean;
+}
+
+int merge_recursive(struct merge_options *opt,
+		    struct commit *h1,
+		    struct commit *h2,
+		    struct commit_list *ca,
+		    struct commit **result)
+{
+	int clean;
+
+	assert(opt->ancestor == NULL);
+
+	if (merge_start(opt, repo_get_commit_tree(opt->repo, h1)))
+		return -1;
+	clean = merge_recursive_internal(opt, h1, h2, ca, result);
+	merge_finalize(opt);
+
+	return clean;
+}
+
 static struct commit *get_ref(struct repository *repo, const struct object_id *oid,
 			      const char *name)
 {