sha1-file.c: don't freshen cruft packs

We don't bother to freshen objects stored in a cruft pack individually by updating the `.mtimes` file. This is because we can't portably `mmap` and write into the middle of a file (i.e., to update the mtime of just one object). Instead, we would have to rewrite the entire `.mtimes` file which may incur some wasted effort especially if there a lot of cruft objects and they are freshened infrequently. Instead, force the freshening code to avoid an optimizing write by writing out the object loose and letting it pick up a current mtime. This works because we prefer the mtime of the loose copy of an object when both a loose and packed one exist (whether or not the packed copy comes from a cruft pack or not). This could certainly do with a test and/or be included earlier in this series/PR, but I want to wait until after I have a chance to clean up the overly-repetitive nature of the cruft pack tests in general. Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>