handle_revision_arg: record paths for pending objects

handle_revision_arg: record paths for pending objects

If the revision parser sees an argument like tree:path, we
parse it down to the correct blob (or tree), but throw away
the "path" portion. Let's ask get_sha1_with_context() to
record it, and pass it along in the pending array.

This will let programs like git-diff which rely on the
revision-parser show more accurate paths.

Note that the implementation is a little tricky; we have to
make sure we free oc.path in all code paths. For handle_dotdot(),
we can piggy-back on the existing cleanup-wrapper pattern.
The real work happens in handle_dotdot_1(), but the
handle_dotdot() wrapper makes sure that the path is freed no
matter how we exit the function (and for that reason we make
sure that the object_context struct is zero'd, so if we fail
to even get to the get_sha1_with_context() call, we just end
up calling free(NULL)).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>