reset --keep: only write index file once

"git reset --keep" calls reset_index_file() twice, first doing a
two-way merge to the target revision, updating the index and worktree,
and then resetting the index. After each call, we write the index
file.

In the unlikely event that the second call to reset_index_file()
fails, the index will have been merged to the target revision, but
HEAD will not be updated, leaving the user with a dirty index.

By moving the locking, writing and committing out of
reset_index_file() and into the caller, we can avoid writing the index
twice, thereby making the sure we don't end up in the half-way reset
state. As a bonus, we speed up "git reset --keep" a little on the
linux-2.6 repo (best of five, warm cache):

        Before      After
real    0m0.315s    0m0.296s
user    0m0.290s    0m0.280s
sys     0m0.020s    0m0.010s

Signed-off-by: Martin von Zweigbergk <martinvonz@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/reset.c

@@ -38,14 +38,12 @@ static inline int is_merge(void)
 	return !access(git_path("MERGE_HEAD"), F_OK);
 }
 
-static int reset_index_file(const unsigned char *sha1, int reset_type, int quiet)
+static int reset_index(const unsigned char *sha1, int reset_type, int quiet)
 {
 	int nr = 1;
-	int newfd;
 	struct tree_desc desc[2];
 	struct tree *tree;
 	struct unpack_trees_options opts;
-	struct lock_file *lock = xcalloc(1, sizeof(struct lock_file));
 
 	memset(&opts, 0, sizeof(opts));
 	opts.head_idx = 1;
@@ -67,8 +65,6 @@ static int reset_index_file(const unsigned char *sha1, int reset_type, int quiet
 		opts.reset = 1;
 	}
 
-	newfd = hold_locked_index(lock, 1);
-
 	read_cache_unmerged();
 
 	if (reset_type == KEEP) {
@@ -91,10 +87,6 @@ static int reset_index_file(const unsigned char *sha1, int reset_type, int quiet
 		prime_cache_tree(&active_cache_tree, tree);
 	}
 
-	if (write_cache(newfd, active_cache, active_nr) ||
-	    commit_locked_index(lock))
-		return error(_("Could not write new index file."));
-
 	return 0;
 }
 
@@ -341,9 +333,16 @@ int cmd_reset(int argc, const char **argv, const char *prefix)
 		die_if_unmerged_cache(reset_type);
 
 	if (reset_type != SOFT) {
-		int err = reset_index_file(sha1, reset_type, quiet);
+		struct lock_file *lock = xcalloc(1, sizeof(struct lock_file));
+		int newfd = hold_locked_index(lock, 1);
+		int err = reset_index(sha1, reset_type, quiet);
 		if (reset_type == KEEP && !err)
-			err = reset_index_file(sha1, MIXED, quiet);
+			err = reset_index(sha1, MIXED, quiet);
+		if (!err &&
+		    (write_cache(newfd, active_cache, active_nr) ||
+		     commit_locked_index(lock))) {
+			err = error(_("Could not write new index file."));
+		}
 		if (err)
 			die(_("Could not reset index file to revision '%s'."), rev);
 	}