The old `git_fileops_prettify_path` has been replaced with
`git_path_prettify`. This is a much simpler method that uses the OS's
`realpath` call to obtain the full path for directories and resolve
symlinks.

The `realpath` syscall is the original POSIX call in Unix system and
an emulated version under Windows using the Windows API.

Cleaned up the structure of the whole OS-abstraction layer.

fileops.c now contains a set of utility methods for file management used
by the library. These are abstractions on top of the original POSIX
calls.

There's a new file called `posix.c` that contains
emulations/reimplementations of all the POSIX calls the library uses.
These are prefixed with `p_`. There's a specific posix file for each
platform (win32 and unix).

All the path-related methods have been moved from `utils.c` to `path.c`
and have their own prefix.