Merge branch 'jc/doc-gc-prune-now' into maint

"git gc" is safe to run anytime only because it has the built-in
grace period to protect young objects.  In order to run with no
grace period, the user must make sure that the repository is
quiescent.

* jc/doc-gc-prune-now:
Documentation/gc: warn against --prune=<now>

diff --cc Documentation/git-gc.txt
Simple merge