When doing pip install from a git working directory, pip moves the
working tree to a temporary folder.

The %TEMP% environment variable may contain short path names, e.g.
"C:\Users\COSIMO~1.LUP\AppData\Local\Temp".

However, when calling `GitWorkdir.from_potential_worktree`, the command
`git rev-parse --show-toplevel` always returns a path with long names.
Because of this, the root and 'real' working directory compare different
from each other, even if they are actually the same, and the method will
return None.

This adds a new function in utils.py module to get the long path name using
the Windows API (via ctypes).

The 'g' prefix is a 'git describe' specific thing, which is that we try
to use first. In the fallback 'git rev-parse' is used to we need to add
the 'g' manually.

Previously, three git commands were used to gather all required info.
Now all info is gathered from a single 'git describe' call in all usual
cases, and only if that fails fallbacks are used.

The --dirty option was already present before commit 929a5d7, but it was
removed to address [1], which turns out to not actually be necessary as
we can try "git describe --dirty" first and then eventually use the
fallback.

[1] https://github.com/pypa/setuptools_scm/issues/86