bug#27787: 26.0.50; Add a way to refresh package list based on last update time

[Mohammed Sadiq <sadiq@sadiqpk.org>]

It would be nice if there is a defun that would update
package list based on last successfull update.

Eg.: Update package list if last update was before 7 days, or 10 hours,
etc.


Or at least, a function that would return the time of last successful
update would be nice.

Thanks


In GNU Emacs 26.0.50 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.22.11)
 of 2017-07-15 built on rose
Repository revision: 511fe21b88f7468a5232434b556f886fa985a5aa
Windowing system distributor 'The X.Org Foundation', version 11.0.11902000
System Description:	Debian GNU/Linux 9.0 (stretch)

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Making completion list... [2 times]

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Memory information:
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