commit: allow partial commits with relative paths

commit: allow partial commits with relative paths

Commit 8894d53580 (commit: allow partial commits with relative paths,
2011-07-30) ensured that partial commits were allowed when a user
supplies a relative pathspec but then this was regressed in 5879f5684c
(remove prefix argument from pathspec_prefix, 2011-09-04) when the
prefix argument to 'pathspec_prefix' removed and the 'list_paths'
function wasn't properly adjusted to cope with the change, resulting in
over-eager pruning of the tree that is overlayed on the index.

This fixes the regression and adds a regression test so this can be
prevented in the future.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>