Check the can_diff and can_merge functions before deciding whether
to add the tool to the available/unavailable lists. This makes
"--tool-help" context-sensitive so that "git mergetool --tool-help"
displays merge tools only and "git difftool --tool-help" displays
diff tools only.
Signed-off-by: David Aguilar <davvid@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>