last-modified: support sparse checkouts

last-modified: support sparse checkouts

In a sparse checkout, a user might want to run `last-modified` on a
directory outside the worktree.

And even in non-sparse checkouts, a user might need to run that command
on a directory that does not exist in the worktree.

These use cases should be supported via the `--` separator between
revision and file arguments, which is even advertised in the
documentation. This patch fixes a tiny bug that prevents that from
working.

This fixes https://github.com/git-for-windows/git/issues/5978

Signed-off-by: Johannes Schindelin <redacted>
Acked-by: Derrick Stolee <redacted>
Acked-by: Toon Claes <redacted>
Signed-off-by: Junio C Hamano <redacted>