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