tests/test-diff-subdir.t

worker: rewrite error handling so os._exit covers all cases

Previously the worker error handling is like:

    pid = os.fork()   --+
    if pid == 0:        |
        ....            | problematic
        ....          --+
        try:          --+
            ....        | worker error handling
                      --+

If a signal arrives when Python is executing the "problematic" lines, an
external error handling (dispatch.py) will take over the control flow and
it's no longer guaranteed "os._exit" is called (see 86cd09bc13ba for why it
is necessary).

This patch rewrites the error handling so it covers all possible code paths
for a worker even during fork.

Note: "os.getpid() == parentpid" is used to test if the process is parent or
not intentionally, instead of checking "pid", because "pid = os.fork()" may
be not atomic - it's possible that that a signal hits the worker before the
assignment completes [1].  The newly added test replaces "os.fork" to
exercise that extreme case.

[1]: CPython compiles "pid = os.fork()" to 2 byte codes: "CALL_FUNCTION" and
"STORE_FAST", so it's probably not atomic:

    def f():
        pid = os.fork()

    dis.dis(f)
      2           0 LOAD_GLOBAL              0 (os)
                  3 LOAD_ATTR                1 (fork)
                  6 CALL_FUNCTION            0
                  9 STORE_FAST               0 (pid)
                 12 LOAD_CONST               0 (None)
                 15 RETURN_VALUE

  $ hg init

  $ mkdir alpha
  $ touch alpha/one
  $ mkdir beta
  $ touch beta/two

  $ hg add alpha/one beta/two
  $ hg ci -m "start"

  $ echo 1 > alpha/one
  $ echo 2 > beta/two

everything

  $ hg diff --nodates
  diff -r 7d5ef1aea329 alpha/one
  --- a/alpha/one
  +++ b/alpha/one
  @@ -0,0 +1,1 @@
  +1
  diff -r 7d5ef1aea329 beta/two
  --- a/beta/two
  +++ b/beta/two
  @@ -0,0 +1,1 @@
  +2

beta only

  $ hg diff --nodates beta
  diff -r 7d5ef1aea329 beta/two
  --- a/beta/two
  +++ b/beta/two
  @@ -0,0 +1,1 @@
  +2

inside beta

  $ cd beta
  $ hg diff --nodates .
  diff -r 7d5ef1aea329 beta/two
  --- a/beta/two
  +++ b/beta/two
  @@ -0,0 +1,1 @@
  +2

relative to beta

  $ cd ..
  $ hg diff --nodates --root beta
  diff -r 7d5ef1aea329 two
  --- a/two
  +++ b/two
  @@ -0,0 +1,1 @@
  +2

inside beta

  $ cd beta
  $ hg diff --nodates --root .
  diff -r 7d5ef1aea329 two
  --- a/two
  +++ b/two
  @@ -0,0 +1,1 @@
  +2

  $ cd ..