Don't overwrite y-or-n-p prompt by message from asynchronous man (bug#19064)

* lisp/man.el (Man-bgproc-sentinel): Postpone displaying the message
and deleting the buffer until exiting the code block with
with-current-buffer.  Use minibuffer-message to display message
to handle possibly active minibuffer.
1 job for master in 90 minutes and 2 seconds (queued for 6 seconds)
Status Job ID Name Coverage
failed #4173


Name Stage Failure
test-all Test The script exceeded the maximum execution time set for the job
make -C test check-expensive
make[1]: Entering directory '/builds/emacs/emacs/test'
rm -f ./*.tmp
make[2]: Entering directory '/builds/emacs/emacs/test'
make[3]: Entering directory '/builds/emacs/emacs/test'
ELC lisp/net/tramp-tests.elc
GEN lisp/net/tramp-tests.log
Pulling docker image gitlab/gitlab-runner-helper:x86_64-d0b76032 ...
ERROR: Job failed: execution took longer than 1h30m0s seconds