Use a temporary index for git commit --interactive

Change the behaviour of git commit --interactive so that when you abort
the commit (by leaving the commit message empty) the index remains
unchanged.

Hitherto an aborted commit --interactive has added the selected hunks to
the index regardless of whether the commit succeeded or not.

Signed-off-by: Conrad Irwin <conrad.irwin@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-commit.txt

@@ -41,7 +41,8 @@ The content to be added can be specified in several ways:
 
 5. by using the --interactive switch with the 'commit' command to decide one
    by one which files should be part of the commit, before finalizing the
-   operation.  Currently, this is done by invoking 'git add --interactive'.
+   operation.  Currently, this is done by invoking 'git add --interactive'
+   on a temporary index.
 
 The `--dry-run` option can be used to obtain a
 summary of what is included by any of the above for the next

builtin/commit.c

@@ -336,18 +336,11 @@ static char *prepare_index(int argc, const char **argv, const char *prefix, int
 	int fd;
 	struct string_list partial;
 	const char **pathspec = NULL;
+	char *old_index_env = NULL;
 	int refresh_flags = REFRESH_QUIET;
 
 	if (is_status)
 		refresh_flags |= REFRESH_UNMERGED;
-	if (interactive) {
-		if (interactive_add(argc, argv, prefix) != 0)
-			die(_("interactive add failed"));
-		if (read_cache_preload(NULL) < 0)
-			die(_("index file corrupt"));
-		commit_style = COMMIT_AS_IS;
-		return get_index_file();
-	}
 
 	if (*argv)
 		pathspec = get_pathspec(prefix, argv);
@@ -355,6 +348,33 @@ static char *prepare_index(int argc, const char **argv, const char *prefix, int
 	if (read_cache_preload(pathspec) < 0)
 		die(_("index file corrupt"));
 
+	if (interactive) {
+		fd = hold_locked_index(&index_lock, 1);
+
+		refresh_cache_or_die(refresh_flags);
+
+		if (write_cache(fd, active_cache, active_nr) ||
+		    close_lock_file(&index_lock))
+			die(_("unable to create temporary index"));
+
+		old_index_env = getenv(INDEX_ENVIRONMENT);
+		setenv(INDEX_ENVIRONMENT, index_lock.filename, 1);
+
+		if (interactive_add(argc, argv, prefix) != 0)
+			die(_("interactive add failed"));
+
+		if (old_index_env && *old_index_env)
+			setenv(INDEX_ENVIRONMENT, old_index_env, 1);
+		else
+			unsetenv(INDEX_ENVIRONMENT);
+
+		discard_cache();
+		read_cache_from(index_lock.filename);
+
+		commit_style = COMMIT_NORMAL;
+		return index_lock.filename;
+	}
+
 	/*
 	 * Non partial, non as-is commit.
 	 *