server: externalize application creation

commands: document and test hg clone update priority

rebase: improve help text - add a paragraph about default dest/source changesets - option help: say "changeset" not "revision" - option help: explain -b/--base better - clarify that -a/--abort and -c/--continue are different from the other options

serve: fix port config

merge with other head, will show bug in hg serve

test-serve: Show if port config and option are correctly used this will break in d3f27d15c9cb

drop (default: 8000), non-zero default is automatically added to help

make expression shorter, now the line fits into 80 chars

serve: fix options recording, trailing whitespace

serve: allow --port=0 to specify "server chooses the port number"