Fix test failure on Windows

This is related to how File.toString uses the path separator of the system
rather than the separator used to construct the file.
Allow overriding the git command with an environment variable

This is needed on Windows machines that have Cygwin Git installed. Cygwin Git
does not work because Java generates Windows-formatted paths with the File
class, but Cygwin Git only supports Unix-style paths. To work around this, I
installed vanilla Windows Git and set HESOGIT to that copy of git.exe. The
vanilla Windows Git supports Windows-style paths.