store: use fixed length hex hashes for dir names

[N-R-K]

mostly cosmetics. having fixed width dirnames results in a nicer listing of the directory. it also matches usual expectation of hashes being stringified in hex format.

[cdown]

I'm not sure I understand the justification, could you go over it more?

[N-R-K]

With the current naming, directories would look something like this:

With fixed-width hex naming, they all have the same width and appear nicer when listing (or when viewed thru a file manager etc).

It's entirely a cosmetic/aesthetic thing. Figured I'd raise it before there's a stable version released in case you like it, otherwise I can just keep the patch locally if you don't.

[cdown]

I was on the fence because while it does make hashes more uniform, it also adds (ok, not much) more work and code. I do think it looks better and is also visually more obvious when seeing clip hashes in CM_DEBUG, though, so let's do it. Thanks!