Fixes to gnus-agent-retrieve-headers

* lisp/gnus/gnus-agent.el (gnus-agent-retrieve-headers): The
sort->append call was altering fetched-headers in-place. Also, we
should explicitly sort-numeric-fields in this
function. gnus-agent-check-overview-buffer will do this, but treats it
as a failure mode and will make a backup of the overview file.
1 job for girzel/gnus-headers in 61 minutes and 6 seconds (queued for 2 seconds)
Status Job ID Name Coverage
failed #10391


Name Stage Failure
test-all Test
make[2]: *** [check-doit] Error 2
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:289: recipe for target 'check-expensive' failed
make[1]: *** [check-expensive] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
Makefile:966: recipe for target 'check-expensive' failed
make: *** [check-expensive] Error 2
Cleaning up file based variables
ERROR: Job failed: exit code 1