Merge branch 'dl/push-missing-object-error' into maint-2.51

Merge branch 'dl/push-missing-object-error' into maint-2.51

"git push" had a code path that led to BUG() but it should have
been a die(), as it is a response to a usual but invalid end-user
action to attempt pushing an object that does not exist.
cf. <xmqqo6spiyqp.fsf@gitster.g>

* dl/push-missing-object-error:
  remote.c: convert if-else ladder to switch
  remote.c: remove BUG in show_push_unqualified_ref_name_error()
  t5516: remove surrounding empty lines in test bodies