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

diff --cc builtin/diff.c
Simple merge

diff --cc contrib/completion/git-completion.bash
index aea30d74a058ff1339f64173be0cbec84c0b6a46,679d1ec8a80c0fc2c587532bdbf0e1fcf9b567e5..36f5a91c7a9859c2fa9a976b0f663b2c7fb31cf8
--- 1/contrib/completion/git-completion.bash
--- 2/contrib/completion/git-completion.bash
+++ b/contrib/completion/git-completion.bash
@@@ -1695,9 -1689,12 +1695,13 @@@ __git_diff_common_options="--stat --num
                        --submodule --submodule= --ignore-submodules
                        --indent-heuristic --no-indent-heuristic
                        --textconv --no-textconv
 +                      --patch --no-patch
  "
  
+ __git_diff_difftool_options="--cached --staged --pickaxe-all --pickaxe-regex
+                       --base --ours --theirs --no-index --relative --merge-base
+                       $__git_diff_common_options"
+ 
  _git_diff ()
  {
        __git_has_doubledash && return

diff --cc diff-lib.c
Simple merge

diff --cc diff.h
Simple merge