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>