ref-filter: implement %(if:equals=<string>) and %(if:notequals=<string>)

Implement %(if:equals=<string>) wherein the if condition is only
satisfied if the value obtained between the %(if:...) and %(then) atom
is the same as the given '<string>'.

Similarly, implement (if:notequals=<string>) wherein the if condition
is only satisfied if the value obtained between the %(if:...) and
%(then) atom is different from the given '<string>'.

This is done by introducing 'if_atom_parser()' which parses the given
%(if) atom and then stores the data in used_atom which is later passed
on to the used_atom of the %(then) atom, so that it can do the required
comparisons.

Add tests and documentation for the same.

Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Matthieu Moy <matthieu.moy@grenoble-inp.fr>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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