mergetool--lib: don't call "exit" in setup_tool

mergetool--lib: don't call "exit" in setup_tool

This will make it easier to use setup_tool in places where we expect
that the selected tool will not support the current mode.

We need to introduce a new return code for setup_tool to differentiate
between the case of "the selected tool is invalid" and "the selected
tool is not a built-in" since we must call setup_tool when a custom
'merge.<tool>.path' is configured for a built-in tool but avoid failing
when the configured tool is not a built-in.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>