tests/test-convert-bzr-114.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

#require bzr bzr114

  $ . "$TESTDIR/bzr-definitions"

The file/directory replacement can only be reproduced on
bzr >= 1.4. Merge it back in test-convert-bzr-directories once
this version becomes mainstream.
replace file with dir

  $ mkdir test-replace-file-with-dir
  $ cd test-replace-file-with-dir
  $ bzr init -q source
  $ cd source
  $ echo d > d
  $ bzr add -q d
  $ bzr commit -q -m 'add d file'
  $ rm d
  $ mkdir d
  $ bzr add -q d
  $ bzr commit -q -m 'replace with d dir'
  $ echo a > d/a
  $ bzr add -q d/a
  $ bzr commit -q -m 'add d/a'
  $ cd ..
  $ hg convert source source-hg
  initializing destination source-hg repository
  scanning source...
  sorting...
  converting...
  2 add d file
  1 replace with d dir
  0 add d/a
  $ manifest source-hg tip
  % manifest of tip
  644   d/a
  $ cd source-hg
  $ hg update
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ cd ../..