Merge branch 'da/mergetool-temporary-directory'

Allow a temporary directory specified to be used while running "git
mergetool" backend.

* da/mergetool-temporary-directory:
  t7610-mergetool: add test cases for mergetool.writeToTemp
  mergetool: add an option for writing to a temporary directory

diff --cc Documentation/config.txt
Simple merge

diff --cc git-mergetool.sh
Simple merge

diff --cc t/t7610-mergetool.sh
Simple merge