add: warn when asked to update SKIP_WORKTREE entries

add: warn when asked to update SKIP_WORKTREE entries

`git add` already refrains from updating SKIP_WORKTREE entries, but it
silently exits with zero code when it is asked to do so. Instead, let's
warn the user and display a hint on how to update these entries.

Note that we only warn the user whey they give a pathspec item that
matches no eligible path for updating, but it does match one or more
SKIP_WORKTREE entries. A warning was chosen over erroring out right away
to reproduce the same behavior `add` already exhibits with ignored
files. This also allow users to continue their workflow without having
to invoke `add` again with only the eligible paths (as those will have
already been added).

Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
Signed-off-by: Junio C Hamano <gitster@pobox.com>