git stash takes excessively long when many untracked files present

[CCing folks involved in the recent "stash-refuse-to-kill" merge.]

I keep portions of my home directory in git.  I tried to "git stash"
some local changes, and it ran for several minutes with no progress.  ps
showed that it was running "git ls-files --killed", which was taking
100% CPU, and occasionally reading the disk very slowly.

strace shows that git ls-files --killed is doing a full recursive
enumeration of my entire home directory.  That's a Really Bad Idea:

~$ find | wc -l
3248997
~$ find -type d | wc -l
350680

Not only that, but it also appears to be attempting to stat and open
several files in every single directory; for instance:

stat(".ccache/1/3/.git", 0x7fff254bc7a0) = -1 ENOENT (No such file or directory)
open(".ccache/1/3/.git/HEAD", O_RDONLY) = -1 ENOENT (No such file or directory)
stat(".ccache/1/3/.git", 0x7fff254bc770) = -1 ENOENT (No such file or directory)
open(".ccache/1/3/.git/packed-refs", O_RDONLY) = -1 ENOENT (No such file or directory)

(Yes, in that order.)

I see a lot of room for optimization here.  Most importantly, git
ls-files --killed really doesn't need to look at any directory entry
unless something in the index would conflict with it.

- Josh Triplett