Merge branch 'jc/parseopt-expiry-errors'

"git gc --prune=nonsense" spent long time repacking and then
silently failed when underlying "git prune --expire=nonsense"
failed to parse its command line.  This has been corrected.

* jc/parseopt-expiry-errors:
parseopt: handle malformed --expire arguments more nicely
gc: do not upcase error message shown with die()

diff --cc builtin/gc.c
Simple merge

diff --cc t/t5304-prune.sh
Simple merge