"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