read-cache.c: do not die if mmap fails

do_read_index() mmaps the index, or tries to die with an error message
on failure. It should call xmmap_gently(), which returns MAP_FAILED,
rather than xmmap(), which dies with its own error message.

An easy way to cause this mmap to fail is by setting $GIT_INDEX_FILE to
a path to a directory and then invoking any command that reads from the
index.
Signed-off-by: NVarun Naik <vcnaik94@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

read-cache.c

@@ -2140,7 +2140,7 @@ int do_read_index(struct index_state *istate, const char *path, int must_exist)
 	if (mmap_size < sizeof(struct cache_header) + the_hash_algo->rawsz)
 		die(_("%s: index file smaller than expected"), path);
 
-	mmap = xmmap(NULL, mmap_size, PROT_READ, MAP_PRIVATE, fd, 0);
+	mmap = xmmap_gently(NULL, mmap_size, PROT_READ, MAP_PRIVATE, fd, 0);
 	if (mmap == MAP_FAILED)
 		die_errno(_("%s: unable to map index file"), path);
 	close(fd);