When you say "!<ENTER>" while running say "git log", you'd confuse
yourself in the resulting shell, that may look as if you took
control back to the original shell you spawned "git log" from but
that isn't what is happening.  To that new shell, we leaked
GIT_PAGER_IN_USE environment variable that was meant as a local
communication between the original "Git" and subprocesses that was
spawned by it after we launched the pager, which caused many
"interesting" things to happen, e.g. "git diff | cat" still paints
its output in color by default.

Stop leaking that environment variable to the pager's half of the
fork; we only need it on "Git" side when we spawn the pager.

* jc/unexport-git-pager-in-use-in-pager:
  pager: do not leak "GIT_PAGER_IN_USE" to the pager