Introduce `git_prefixncmp` that will search up to the first `n`
characters of a string to see if it is prefixed by another string.
This is useful for examining if a non-null terminated character
array is prefixed by a particular substring.

Consolidate the various implementations of `git__prefixcmp` around a
single core implementation and add some test cases to validate its
behavior.

Treat input bytes as unsigned before doing arithmetic on them,
lest we look at some non-ASCII byte (like a UTF-8 character) as a
negative value and perform the comparison incorrectly.