Merge branch 'mh/doc-commit-title-not-subject'

The documentation of "git commit" and "git rebase" now refer to
commit titles as such, not "subject".

* mh/doc-commit-title-not-subject:
  doc: use 'title' consistently

diff --cc Documentation/git-commit.adoc
Simple merge

diff --cc Documentation/git-rebase.adoc
Simple merge