Merge branch 'js/mergetool-optim'

"git mergetool" and its tests now spawn fewer subprocesses. * js/mergetool-optim: mergetool: use shell variable magic instead of `awk` mergetool: dissect strings with shell variable magic instead of `expr` t7610-mergetool: use test_cmp instead of test $(cat file) = $txt t7610-mergetool: do not place pipelines headed by `yes` in subshells