parse-opts: add OPT_FILENAME and transition builtins

parse-opts: add OPT_FILENAME and transition builtins

Commit dbd0f5c (Files given on the command line are relative to $cwd,
2008-08-06) introduced parse_options_fix_filename() as a minimal fix.
OPT_FILENAME is intended to be a more robust fix for the same issue.
OPT_FILENAME and its associated enum OPTION_FILENAME are used to
represent filename options within the parse options API.

This option is similar to OPTION_STRING. If --no is prefixed to the
option the filename is unset. If no argument is given and the default
value is set, the filename is set to the default value. The difference
is that the filename is prefixed with the prefix passed to
parse_options() (or parse_options_start()).

Update git-apply, git-commit, git-fmt-merge-msg, and git-tag to use
OPT_FILENAME with their filename options. Also, rename
parse_options_fix_filename() to fix_filename() as it is no longer
extern.

Signed-off-by: Stephen Boyd <bebarino@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>