ctype classification takes an integer in as returned from getc()
if we just sign extend characters to integers 128-255 will be
misclassified. (255 will become EOF)
Newlib in particular doesn't like this since they uses the value
as an index in a lookup table.

The Microsoft C runtime (MSVCRT) may take a heavy lock on the
locale in order to figure out how the `ctype` functions work.
This is deeply slow. Provide our own to avoid that.

On POSIX, provide emulation for that functionality using the ctype
functions, but compress the return value into a `bool`, and cast the
value to an `unsigned char`.

Use the `git__` ctype functions (`git__isdigit` and friends) instead of
explicitly casting.