Refine pluggable similarity API

Refine pluggable similarity API

This plugs in the three basic similarity strategies for handling
whitespace via internal use of the pluggable API.  In so doing, I
realized that the use of git_buf in the hashsig API was not needed
and actually just made it harder to use, so I tweaked that API as
well.

Note that the similarity metric is still not hooked up in the
find_similarity code - this is just setting out the function that
will be used.