repo_read_index: don't discard the index

Have 'repo_read_index()' behave more like the other read_index family of
functions and don't discard the index if it has already been populated
and instead rely on the quick return of read_index_from which has:

  /* istate->initialized covers both .git/index and .git/sharedindex.xxx */
  if (istate->initialized)
    return istate->cache_nr;

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

repository.c

@@ -235,8 +235,6 @@ int repo_read_index(struct repository *repo)
 {
 	if (!repo->index)
 		repo->index = xcalloc(1, sizeof(*repo->index));
-	else
-		discard_index(repo->index);
 
 	return read_index_from(repo->index, repo->index_file);
 }

repository.h

@@ -92,6 +92,14 @@ extern int repo_submodule_init(struct repository *submodule,
 			       const char *path);
 extern void repo_clear(struct repository *repo);
 
+/*
+ * Populates the repository's index from its index_file, an index struct will
+ * be allocated if needed.
+ *
+ * Return the number of index entries in the populated index or a value less
+ * than zero if an error occured.  If the repository's index has already been
+ * populated then the number of entries will simply be returned.
+ */
 extern int repo_read_index(struct repository *repo);
 
 #endif /* REPOSITORY_H */