mergetools/vimdiff: add vimdiff1 merge tool variant

mergetools/vimdiff: add vimdiff1 merge tool variant

This adds yet another vimdiff/gvimdiff variant and presents conflicts as
a two-way diff between 'LOCAL' and 'REMOTE'. 'MERGED' is not opened
which deviates from the norm so usage text is echoed as a Vim message on
startup that instructs the user with how to proceed and how to abort.

Vimdiff is well-suited to two-way diffs so this is an option for a more
simple, more streamlined conflict resolution. For example: it is
difficult to communicate differences across more than two files using
only syntax highlighting; default vimdiff commands to get and put
changes between buffers do not need the user to manually specify
a source or destination buffer when only using two buffers.

Like other merge tools that directly compare 'LOCAL' with 'REMOTE', this
tool will benefit when paired with the new `mergetool.hideResolved`
setting.

Signed-off-by: Seth House <seth@eseth.com>
Tested-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>