builtin/add: error out when passing untracked path with -u

builtin/add: error out when passing untracked path with -u

When passing untracked path with -u option, it silently succeeds.
There is no error message and the exit code is zero. This is
inconsistent with other instances of git commands where the expected
argument is a known path. In those other instances, we error out when
the path is not known.

Fix this by passing a character array to add_files_to_cache() to
collect the pathspec matching information and report the error if a
pathspec does not match any cache entry. Also add a testcase to cover
this scenario.

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