Use the user's Cygwin home directory if invoked by a Cygwin shell
If we are running within a Cygwin based environment and the user
has given us a HOME environment variable that points to their
Cygwin home directory and they have given us cygpath.exe in PATH
we should translate their Cygwin based HOME directory into a form
that the Windows JRE can read, and use that rather than their own
Windows home directory. This is based under the assumption that
anyone using Cygwin with jgit is most likely using a Cygwin based
Git implementation ad thus will want jgit to play alongside it as
much as possible.
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>