Windows: A minimal implemention of getpwuid().

Windows: A minimal implemention of getpwuid().

getpwuid() is implemented just enough that GIT does not issue errors.
Since the information that it returns is not very useful, users are
required to set up user.name and user.email configuration.

All uses of getpwuid() are like getpwuid(getuid()), hence, the return value
of getuid() is irrelevant and the uid parameter is not even looked at.

Side note: getpwnam() is only used to resolve '~' and '~username' paths,
which is an idiom not known on Windows, hence, we don't implement it.

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>