ref-filter: introduce parsing functions for each valid atom

ref-filter: introduce parsing functions for each valid atom

Parsing atoms is done in populate_value(), this is repetitive and
hence expensive. Introduce a parsing function which would let us parse
atoms beforehand and store the required details into the 'used_atom'
structure for further usage.

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