mq: migrate to context manager for changing dirstate parents

dirstate: introduce new context manager for marking dirstate parent changes

contrib: make editmergeps able to work with notepad++ Notepad++ has a different FIRSTLINE argument, so needs special handling.

contrib: make editmergeps able to work with Windows GUI editors Using Start-Process -Wait makes it wait until the process finishes, which is necesssary for Windows GUI applications. My short testing also demonstrated that it does not hurt with command line vim.

extensions: show deprecation warning for the use of cmdutil.command Since this is a fundamental API for extensions, we set 1-year period until actually removing it.

extensions: prohibit registration of command without using @command (API) Detect the problem earlier for better error indication. I'm tired of teaching users that the mq extension is not guilty but the third-party extension is. https://bitbucket.org/tortoisehg/thg/issues?q=%27norepo%27

extensions: optionally print hint on import failure Test will be added by the next patch.

error: add hint to ProgrammingError As the hint isn't shown by the default exception handler, we need to print it manually. I've copied the "** " style from _exceptionwarning().

registrar: unindent superfluous "if True" block

registrar: switch @command decorator to class It overrides _funcregistrarbase._doregister() since the structure of the command table is quite different.