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