mergetool: add hideResolved configuration

The purpose of a mergetool is to help the user resolve any conflicts
that Git cannot automatically resolve. If there is a conflict that must
be resolved manually Git will write a file named MERGED which contains
everything Git was able to resolve by itself and also everything that it
was not able to resolve wrapped in conflict markers.

One way to think of MERGED is as a two- or three-way diff. If each
"side" of the conflict markers is separately extracted an external tool
can represent those conflicts as a side-by-side diff.

However many mergetools instead diff LOCAL and REMOTE both of which
contain versions of the file from before the merge. Since the conflicts
Git resolved automatically are not present it forces the user to
manually re-resolve those conflicts. Some mergetools also show MERGED
but often only for reference and not as the focal point to resolve the
conflicts.

This adds a `mergetool.hideResolved` flag that will overwrite LOCAL and
REMOTE with each corresponding "side" of a conflicted file and thus hide
all conflicts that Git was able to resolve itself. Overwriting these
files will immediately benefit any mergetool that uses them without
requiring any changes to the tool.

No adverse effects were noted in a small survey of popular mergetools[1]
so this behavior defaults to `true`. However it can be globally disabled
by setting `mergetool.hideResolved` to `false`.

[1] https://www.eseth.org/2020/mergetools.html
    https://github.com/whiteinge/eseth/blob/c884424769fffb05d87afb33b2cf80cecb4044c3/2020/mergetools.md

Original-implementation-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Seth House <seth@eseth.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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