Merge branch 'dl/diff-merge-base'
"git diff A...B" learned "git diff --merge-base A B", which is a
longer short-hand to say the same thing.
* dl/diff-merge-base:
contrib/completion: complete `git diff --merge-base`
builtin/diff-tree: learn --merge-base
builtin/diff-index: learn --merge-base
t4068: add --merge-base tests
diff-lib: define diff_get_merge_base()
diff-lib: accept option flags in run_diff_index()
contrib/completion: extract common diff/difftool options
git-diff.txt: backtick quote command text
git-diff-index.txt: make --cached description a proper sentence
t4068: remove unnecessary >tmp