ref-filter: implement %(if), %(then), and %(else) atoms

Implement %(if), %(then) and %(else) atoms. Used as
%(if)...%(then)...%(end) or %(if)...%(then)...%(else)...%(end). If the
format string between %(if) and %(then) expands to an empty string, or
to only whitespaces, then the whole %(if)...%(end) expands to the string
following %(then). Otherwise, it expands to the string following
%(else), if any. Nesting of this construct is possible.

This is in preparation for porting over `git branch -l` to use
ref-filter APIs for printing.

Add documentation and tests regarding 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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