* 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