bug#36707: [PATCH] Signal user-error on duplicate package refresh

[Stefan Kangas]

In the "*Packages*" buffer (M-x package-list-packages), if I hit "r" many times
it will try to initiate more refreshes even if the first one has not finished
yet.  This is unnecessary and potentially confusing.

This patch makes it check if there is already an refresh in progress and signal
a user-error if there is one.

(I'll send the patch in a separate message once this is assigned a bug
number.)

Thanks,
Stefan Kangas