extdiff: sort files when snapshotting This fixes output stability and is generally filesystem-performance-friendly.

filemerge: split the logic for finding an external tool to its own function This will be used by extdiff in an subsequent patch.

largefiles: simplify check for lack of path arguments Instead of checking for a partial merge by checking that the matches has no files and no patterns, check that it's not an always-matcher. Except for being shorter, it also catches the rare case of an exact-matcher with no files.