am -3: do not let failed merge from completing the error codepath

When "am" was rewritten in C, the codepath for falling back to
three-way merge was mistakenly made to make an internal call to
merge-recursive, disabling the error reporting code for certain
types of errors merge-recursive detects and reports by calling
die().

This is a quick-fix for correctness.  The ideal endgame would be to
replace run_command() in run_fallback_merge_recursive() with a
direct call after making sure that internal call to merge-recursive
does not die().

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/am.c

@@ -1589,17 +1589,45 @@ static int build_fake_ancestor(const struct am_state *state, const char *index_f
 	return 0;
 }
 
+/**
+ * Do the three-way merge using fake ancestor, his tree constructed
+ * from the fake ancestor and the postimage of the patch, and our
+ * state.
+ */
+static int run_fallback_merge_recursive(const struct am_state *state,
+					unsigned char *orig_tree,
+					unsigned char *our_tree,
+					unsigned char *his_tree)
+{
+	struct child_process cp = CHILD_PROCESS_INIT;
+	int status;
+
+	cp.git_cmd = 1;
+
+	argv_array_pushf(&cp.env_array, "GITHEAD_%s=%.*s",
+			 sha1_to_hex(his_tree), linelen(state->msg), state->msg);
+	if (state->quiet)
+		argv_array_push(&cp.env_array, "GIT_MERGE_VERBOSITY=0");
+
+	argv_array_push(&cp.args, "merge-recursive");
+	argv_array_push(&cp.args, sha1_to_hex(orig_tree));
+	argv_array_push(&cp.args, "--");
+	argv_array_push(&cp.args, sha1_to_hex(our_tree));
+	argv_array_push(&cp.args, sha1_to_hex(his_tree));
+
+	status = run_command(&cp) ? (-1) : 0;
+	discard_cache();
+	read_cache();
+	return status;
+}
+
 /**
  * Attempt a threeway merge, using index_path as the temporary index.
  */
 static int fall_back_threeway(const struct am_state *state, const char *index_path)
 {
 	unsigned char orig_tree[GIT_SHA1_RAWSZ], his_tree[GIT_SHA1_RAWSZ],
 		      our_tree[GIT_SHA1_RAWSZ];
-	const unsigned char *bases[1] = {orig_tree};
-	struct merge_options o;
-	struct commit *result;
-	char *his_tree_name;
 
 	if (get_sha1("HEAD", our_tree) < 0)
 		hashcpy(our_tree, EMPTY_TREE_SHA1_BIN);
@@ -1651,22 +1679,11 @@ static int fall_back_threeway(const struct am_state *state, const char *index_pa
 	 * changes.
 	 */
 
-	init_merge_options(&o);
-
-	o.branch1 = "HEAD";
-	his_tree_name = xstrfmt("%.*s", linelen(state->msg), state->msg);
-	o.branch2 = his_tree_name;
-
-	if (state->quiet)
-		o.verbosity = 0;
-
-	if (merge_recursive_generic(&o, our_tree, his_tree, 1, bases, &result)) {
+	if (run_fallback_merge_recursive(state, orig_tree, our_tree, his_tree)) {
 		rerere(state->allow_rerere_autoupdate);
-		free(his_tree_name);
 		return error(_("Failed to merge in the changes."));
 	}
 
-	free(his_tree_name);
 	return 0;
 }