checkout, restore: support the --pathspec-from-file option

Decisions taken for simplicity:
1) For now, `--pathspec-from-file` is declared incompatible with
   `--patch`, even when <file> is not `stdin`. Such use case it not
   really expected.
2) It is not allowed to pass pathspec in both args and file.

`you must specify path(s) to restore` block was moved down to be able to
test for `pathspec.nr` instead, because testing for `argc` is no longer
correct.

`git switch` does not support the new options because it doesn't expect
`<pathspec>` arguments.

Signed-off-by: Alexandr Miloslavskiy <alexandr.miloslavskiy@syntevo.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t9902-completion.sh

@@ -1438,6 +1438,8 @@ test_expect_success 'double dash "git checkout"' '
 	--no-guess Z
 	--no-... Z
 	--overlay Z
+	--pathspec-file-nul Z
+	--pathspec-from-file=Z
 	EOF
 '