Merge branch 'jk/send-email-sender-prompt'

General clean-ups in various areas, originally written to support a patch that later turned out to be unneeded. * jk/send-email-sender-prompt: t9001: check send-email behavior with implicit sender t: add tests for "git var" ident: keep separate "explicit" flags for author and committer ident: make user_ident_explicitly_given static t7502: factor out autoident prerequisite test-lib: allow negation of prerequisites