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.

This moves the similarity metric code out of buf_text and into a
new file.  Also, this implements a different approach to similarity
measurement based on a Rabin-Karp rolling hash where we only keep
the top 100 and bottom 100 hashes.  In theory, that should be
sufficient samples to given a fairly accurate measurement while
limiting the amount of data we keep for file signatures no matter
how large the file is.