Merge branch 'pe/doc-email-env-is-trumped-by-config'

In the precedence order, the environment variable $EMAIL comes
between the built-in default (i.e. taking value by asking the
system's gethostname() etc.) and the user.email configuration
variable; the documentation implied that it is stronger than the
configuration like $GIT_COMMITTER_EMAIL is, which is wrong.

* pe/doc-email-env-is-trumped-by-config:
  git-commit-tree(1): correct description of defaults