mergetools: simplify how we handle "vim" and "defaults"

mergetools: simplify how we handle "vim" and "defaults"

Remove the exceptions for "vim" and "defaults" in the mergetool library
so that every filename in mergetools/ matches 1:1 with the name of a
valid built-in tool.

Define the trivial fallback definition of shell functions in-line in
git-mergetool-lib script, instead of dot-sourcing them from another
file.  The result is much easier to follow.

[jc: squashed in an update from John Keeping as well]

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>