remote-helpers: generate scripts

The same way other python scripts are generated, so the shebang is
replaced by PYTHON_PATH.

Also, cleanup the Makefile.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
build: fix installation of scripts

They need the gitexecdir.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
remote-helpers: rename tests

The .t extension is more standard for sharness tests.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
remote-helpers: allow direct test execution

Previously 'make' was the only option, or manually specifying the
'TEST_DIRECTORY'.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
remote-helpers: add exec-path links

This way we don't have to modify the PATH ourselves and it's easier to
test without 'make'.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>