Introduce close_pack_index to permit replacement

By closing the pack index, a caller can later overwrite the index
with an updated index file, possibly after converting from v1 to
the v2 format.  Because p->index_data is NULL after close, on the
next access the index will be opened again and the other members
will be updated with new data.
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

cache.h

@@ -911,6 +911,7 @@ extern struct packed_git *find_sha1_pack(const unsigned char *sha1,
 
 extern void pack_report(void);
 extern int open_pack_index(struct packed_git *);
+extern void close_pack_index(struct packed_git *);
 extern unsigned char *use_pack(struct packed_git *, struct pack_window **, off_t, unsigned int *);
 extern void close_pack_windows(struct packed_git *);
 extern void unuse_pack(struct pack_window **);

sha1_file.c

@@ -606,6 +606,14 @@ void unuse_pack(struct pack_window **w_cursor)
 	}
 }
 
+void close_pack_index(struct packed_git *p)
+{
+	if (p->index_data) {
+		munmap((void *)p->index_data, p->index_size);
+		p->index_data = NULL;
+	}
+}
+
 /*
  * This is used by git-repack in case a newly created pack happens to
  * contain the same set of objects as an existing one.  In that case
@@ -627,8 +635,7 @@ void free_pack_by_name(const char *pack_name)
 			close_pack_windows(p);
 			if (p->pack_fd != -1)
 				close(p->pack_fd);
-			if (p->index_data)
-				munmap((void *)p->index_data, p->index_size);
+			close_pack_index(p);
 			free(p->bad_object_sha1);
 			*pp = p->next;
 			free(p);