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