build: fix installation of scripts

They need the gitexecdir.

Otherwise the child scripts fail when installing:

  % make -C contrib/remote-helpers install
  install: target ‘$d/libexec/git-core’ is not a directory
  Makefile:513: recipe for target 'install-python-script' failed

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>
remote-helpers: rename tests

The .t extension is more standard for sharness tests.

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

They should be distributed and tested by default.

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