Merge branch 'sb/atomic-push'

"git push" has been taught a "--atomic" option that makes push to
update more than one ref an "all-or-none" affair.

* sb/atomic-push:
Document receive.advertiseatomic
t5543-atomic-push.sh: add basic tests for atomic pushes
push.c: add an --atomic argument
send-pack.c: add --atomic command line argument
send-pack: rename ref_update_to_be_sent to check_to_send_update
receive-pack.c: negotiate atomic push support
receive-pack.c: add execute_commands_atomic function
receive-pack.c: move transaction handling in a central place
receive-pack.c: move iterating over all commands outside execute_commands
receive-pack.c: die instead of error in case of possible future bug
receive-pack.c: shorten the execute_commands loop over all commands

diff --cc Documentation/config.txt
Simple merge

diff --cc Documentation/git-push.txt
Simple merge

diff --cc builtin/push.c
Simple merge

diff --cc builtin/receive-pack.c
Simple merge

diff --cc send-pack.c
Simple merge

diff --cc transport.c
Simple merge