hold_locked_index(): move from lockfile.c to read-cache.c

lockfile.c contains the general API for locking any file. Code
specifically about the index file doesn't belong here.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

lockfile.c

@@ -340,14 +340,6 @@ int commit_lock_file(struct lock_file *lk)
 	return err;
 }
 
-int hold_locked_index(struct lock_file *lk, int die_on_error)
-{
-	return hold_lock_file_for_update(lk, get_index_file(),
-					 die_on_error
-					 ? LOCK_DIE_ON_ERROR
-					 : 0);
-}
-
 void rollback_lock_file(struct lock_file *lk)
 {
 	if (!lk->active)

read-cache.c

@@ -1367,6 +1367,14 @@ static int read_index_extension(struct index_state *istate,
 	return 0;
 }
 
+int hold_locked_index(struct lock_file *lk, int die_on_error)
+{
+	return hold_lock_file_for_update(lk, get_index_file(),
+					 die_on_error
+					 ? LOCK_DIE_ON_ERROR
+					 : 0);
+}
+
 int read_index(struct index_state *istate)
 {
 	return read_index_from(istate, get_index_file());