multi-pack-index: close file descriptor after mmap

The multi-pack-index subsystem was not closing its file descriptor
after memory-mapping the file contents. After this mmap() succeeds,
there is no need to keep the file descriptor open.  In fact, there
is signficant reason to close it so we do not run out of
descriptors.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

midx.h

@@ -6,8 +6,6 @@
 struct multi_pack_index {
 	struct multi_pack_index *next;
 
-	int fd;
-
 	const unsigned char *data;
 	size_t data_len;