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