receive-pack.c: shorten the execute_commands loop over all commands

receive-pack.c: shorten the execute_commands loop over all commands

Make the main "execute_commands" loop in receive-pack easier to read
by splitting out some steps into helper functions. The new helper
'should_process_cmd' checks if a ref update is unnecessary, whether
due to an error having occurred or for another reason. The helper
'warn_if_skipped_connectivity_check' warns if we have forgotten to
run a connectivity check on a ref which is shallow for the client
which would be a bug.

This will help us to duplicate less code in a later patch when we make
a second copy of the "execute_commands" loop.

No functional change intended.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>