Merge branch 'da/difftool'

Merge branch 'da/difftool'

* da/difftool:
  mergetool--lib: simplify API usage by removing more global variables
  Fix misspelled mergetool.keepBackup
  difftool/mergetool: refactor commands to use git-mergetool--lib
  mergetool: use $( ... ) instead of `backticks`
  bash completion: add git-difftool
  difftool: add support for a difftool.prompt config variable
  difftool: add various git-difftool tests
  difftool: move 'git-difftool' out of contrib
  difftool/mergetool: add diffuse as merge and diff tool
  difftool: add a -y shortcut for --no-prompt
  difftool: use perl built-ins when testing for msys
  difftool: remove the backup file feature
  difftool: remove merge options for opendiff, tkdiff, kdiff3 and xxdiff
  git-mergetool: add new merge tool TortoiseMerge
  git-mergetool/difftool: make (g)vimdiff workable under Windows
  doc/merge-config: list ecmerge as a built-in merge tool