Revert "always read stdout and stderr from forked commands"

This reverts commit f743394d2d936f487e430e1c815903ac013ec7c3.

Most Git-Cheetah commands are expected to launch an independent process,
without blocking the calling file manager application.

Signed-off-by: Karsten Blees <blees@dcon.de>
exec.c: redirect unwanted command output to /dev/null

Git-Cheetah redirects and reads stdout / stderr of forked commands only if
it needs to. Otherwise this output simply goes to stdout / stderr of the
calling file manager process. This is OK for GUI-based file managers (e.g.
Windows Explorer), as there is no console.

For text-based file managers (e.g. Far Manager) this is a problem, as such
unwanted console output messes up the UI.

Redirect stdout / stderr of forked commands to /dev/null if Git-Cheetah
doesn't need it.

Signed-off-by: Karsten Blees <blees@dcon.de>
Merge pull request #14 from kblees/kb/redirect-output-to-null

Fix menu commands blocking the file manager process