[GSoC] [RFC] Unify ref-filter formats with other --pretty formats

Reference: https://git.github.io/SoC-2019-Ideas/#unify-ref-filter-formats-with-other---pretty-formats

I have spent some time with both pretty.* and ref-filter.*

First off, we are aiming to reuse ref-filter, so avoiding any sort of re-implementation is recommended.

Now, coming to pretty.* and ref-filter.*

suppose, a function named xyz() in ref-filter.c seems like it could be reused in pretty.c. since ref-filter doesn't use any struct of pretty.c. The xyz() function in its original form is not useful for pretty.c. So now, in order for the xyz() function to be useful in pretty.c. Function xyz() should be using structs of pretty.*

now, if we make xyz() use the pretty.* structs, then its re-implementation and not reusing. its like keeping two different functions one for ref-filter and another for pretty.*. which is what is already happening.

please provide any starting point for reusing ref-filter. i don't see any in pretty.*. reusing ref-filter specifically in pretty.* is not the motive. please point out any file in entire code base, that you may feel can reuse some ref-filter logic.