diff.tool or merge.tool should not be required when using --extcmd

[revane]

diff.tool or merge.tool is only necessary if using git-mergetool--lib to execute a merge tool. If you're running an external command this information is not required. I suggest changing the logic to test for presence of diff.tool or merge.tool only once args are parsed and --extcmd is not present.

If diff.tool is not present you get an annoying message from get_merge_tool(). This could be avoided if we only call this function if --extcmd is not present.