builtin/commit: error out when passing untracked path with -i

builtin/commit: error out when passing untracked path with -i

When we provide a pathspec which does not match any tracked path
alongside --include, we do not error like without --include. If there
is something staged, it will commit the staged changes and ignore the
pathspec which does not match any tracked path. And if nothing is
staged, it will print the status. Exit code is 0 in both cases (unlike
without --include). This is also described in the TODO comment before
the relevant testcase.

Fix this by passing a character array to add_files_to_cache() to
collect the pathspec matching information and error out if the given
path is untracked. Also, amend the testcase to check for the error
message and remove the TODO comment.

Signed-off-by: Ghanshyam Thakkar <shyamthakkar001@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>