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>