git-gui: handle textconv filter on Windows and in development

When developing/testing we run git-gui.sh directly and the makefile
configured variables are not properly set. Configure the new shellpath
accessor to handle this case.

On Windows we may not find the shell so in this case revert to simply
executing the filter command without the shell intermediate.

Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>