git: fix numerous bugs in git_quote and git_unquote

git: fix numerous bugs in git_quote and git_unquote

git always quotes with leading zeros to ensure the octal
representation is 3 characters long.  We enforce that to match
low ASCII characters (e.g. [x01-\x06]) that don't need the
range provided by 3 characters.

git_unquote now does a single pass so it won't get fooled by
decoded backslashes into parsing a digit as an octal character.
git_unquote is also capped to "\377" so we don't overflow a
byte.