require "annex/objects/" before key in pointer files

require "annex/objects/" before key in pointer files

This removes ambiguity, because while someone might have "WORM--foo" in a
file that's not intended to be a git-annex pointer file,
"annex/objects/WORM--foo" is less likely.

Also, 664cc987e806800ef46794f356003321170c1060 had a caveat about symlink
targets being parsed as pointer files, and now the same parser is used for
both.

I did not include any hash directories before the key in the pointer file,
as they're not needed. However, if they were included, the parser would
still work ok.