ref-filter: introduce align_atom_parser()

ref-filter: introduce align_atom_parser()

Introduce align_atom_parser() which will parse an 'align' atom and
store the required alignment position and width in the 'used_atom'
structure for further usage in populate_value().

Since this patch removes the last usage of match_atom_name(), remove
the function from ref-filter.c.

Helped-by: Eric Sunshine <redacted>
Helped-by: Jeff King <redacted>
Helped-by: Ramsay Jones <redacted>
Signed-off-by: Karthik Nayak <redacted>
Reviewed-by: Eric Sunshine <redacted>
Signed-off-by: Junio C Hamano <redacted>