add infrastructure for read-only file system level caches

Add a macro to mark code sections that only read from the file system,
along with a config option and documentation.

This facilitates implementation of relatively simple file system level
caches without the need to synchronize with the file system.

Enable read-only sections for 'git status' and preload_index.

Signed-off-by: Karsten Blees <blees@dcon.de>

builtin/commit.c

@@ -1361,6 +1361,7 @@ int cmd_status(int argc, const char **argv, const char *prefix)
 		       PATHSPEC_PREFER_FULL,
 		       prefix, argv);
 
+	enable_fscache(1);
 	read_cache_preload(&s.pathspec);
 	refresh_index(&the_index, REFRESH_QUIET|REFRESH_UNMERGED, &s.pathspec, NULL, NULL);