pathspec.c: support adding prefix magic to a pathspec with mnemonic magic

pathspec.c: support adding prefix magic to a pathspec with mnemonic magic

Back in 233c3e6 (parse_pathspec: preserve prefix length via
PATHSPEC_PREFIX_ORIGIN - 2013-07-14), parse_pathspec() is taught to
save prefix length as a dynamic magic. This is needed when the
pathspec is passed to another process and and prefix lenght would be
lost.

Back then we support two cases. If the pathspec is normal, e.g. "abc",
we simply add the prefix to become ":(prefix:2)abc". If the pathspec
contains long magic, e.g. ":(foo,bar)abc" then we turn it to
":(foo,bar,prefix:2)abc". We do not support prefixing on short form,
because the only supported mnemonic '/' disappears after the the
preprocessing steps.

With the introduction of exclude magic with mnemonic '!', we need to
add support for the short form case so that ':!abc' becomes
':(exclude,prefix:2)abc'. Without this, it will break

    cd Documentation
    git add -p -- . ':!technical'

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>