Allow overriding the home directory via the HOME variable

[dscho]

In Git for Windows, it is a well-established technique to use the
$HOME variable to define where the current user's home directory is,
falling back to $HOMEDRIVE$HOMEPATH and $USERPROFILE.

This strategy is particular important when Cygwin, or command-line
programs depending on the HOME variable, cannot cope with the Windows'
idea of the user's home directory e.g. when it is set to a hidden
directory via an UNC path (\share\some\hidden\folder$).

Of course this strategy needs to be opt-in. For that reason, this
strategy is activated via the env keyword in the db_home line in
/etc/nsswitch.conf.

Signed-off-by: Johannes Schindelin johannes.schindelin@gmx.de

[dscho]

This PR is intended to address git-for-windows/git#108 (and also git-for-windows/git#100).