[PATCH v2] generalizing sorted-array handling

This is a complete rewrite of the previous series, following a
suggestion by Jonathan Nieder.  It is much more generic than the
previous one, and removes existing duplicated code in diffcore-rename.

I also had a quick look at other binary-search occurences in the code
that would share the same search code.  That revealed
index_name_pos(): not sure it is a good idea to make it use this API -
it would require changes all over the code.  I still include a
possibly-useful additional API for similar cases.