pathspec: add new function to parse file

pathspec: add new function to parse file

This will be used to support the new option '--pathspec-from-file' in
`git add`, `git-commit`, `git reset` etc.

Note also that we specifically handle CR/LF line endings to support
Windows better.

To simplify code, file is first parsed into `argv_array`. This allows
to avoid refactoring `parse_pathspec()`.

I considered adding `nul_term_line` to `flags` instead, but decided
that it doesn't fit there.

The new code is mostly taken from `cmd_update_index()` and
`split_mail_conv()`.

Co-authored-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Alexandr Miloslavskiy <alexandr.miloslavskiy@syntevo.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>