Merge branch 'ss/mergetool--lib'

* ss/mergetool--lib:
mergetool--lib: Add Beyond Compare 3 as a tool
mergetool--lib: Sort tools alphabetically for easier lookup

diff --cc Documentation/git-difftool.txt
Simple merge

diff --cc Documentation/git-mergetool.txt
Simple merge

diff --cc contrib/completion/git-completion.bash
Simple merge

diff --cc git-mergetool--lib.sh
Simple merge