Introduce `git__getenv` which is a UTF-8 aware `getenv` everywhere.
Make `cl_getenv` use this to keep consistent memory handling around
return values (free everywhere, as opposed to only some platforms).

Some brain damaged tolower() implementations appear to want to
take the locale into account, and this may require taking some
insanely aggressive lock on the locale and slowing down what should
be the most trivial of trivial calls for people who just want to
downcase ASCII.

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.

Disallow:
 1. paths with trailing dot
 2. paths with trailing space
 3. paths with trailing colon
 4. paths that are 8.3 short names of .git folders ("GIT~1")
 5. paths that are reserved path names (COM1, LPT1, etc).
 6. paths with reserved DOS characters (colons, asterisks, etc)

These paths would (without \\?\ syntax) be elided to other paths - for
example, ".git." would be written as ".git".  As a result, writing these
paths literally (using \\?\ syntax) makes them hard to operate with from
the shell, Windows Explorer or other tools.  Disallow these.

This makes the index iterator honor the GIT_ITERATOR_IGNORE_CASE
and GIT_ITERATOR_DONT_IGNORE_CASE flags without modifying the
index data itself.  To take advantage of this, I had to export a
number of the internal index entry comparison functions.  I also
wrote some new tests to exercise the capability.