git-gui: Handle workdir detection when CYGWIN=nowinsymlinks

git-gui: Handle workdir detection when CYGWIN=nowinsymlinks

If the user has put nowinsymlinks into their CYGWIN environment
variable any symlinks created by a Cygwin process (e.g. ln -s)
will not have the ".lnk" suffix.  In this case workdir is still
a workdir, but our detection of looking for "info.lnk" fails
as the symlink is actually a normal file called "info".

Instead we just always use Cygwin's test executable to see if
info/exclude is a file.  If it is, we assume from there on it
can be read by git-ls-files --others and is thus safe to use
on the command line.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>