Merge branch 'jh/verify-index-checksum-only-in-fsck'

The index file has a trailing SHA-1 checksum to detect file
corruption, and historically we checked it every time the index
file is used.  Omit the validation during normal use, and instead
verify only in "git fsck".

* jh/verify-index-checksum-only-in-fsck:
  read-cache: force_verify_index_checksum

builtin/fsck.c

@@ -771,6 +771,7 @@ int cmd_fsck(int argc, const char **argv, const char *prefix)
 	}
 
 	if (keep_cache_objects) {
+		verify_index_checksum = 1;
 		read_cache();
 		for (i = 0; i < active_nr; i++) {
 			unsigned int mode;