tests/test-i18n.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

(Translations are optional)

#if gettext no-outer-repo

Test that translations are compiled and installed correctly.

Default encoding in tests is "ascii" and the translation is encoded
using the "replace" error handler:

  $ LANGUAGE=pt_BR hg tip
  abortado: n?o foi encontrado um reposit?rio em '$TESTTMP' (.hg n?o encontrado)!
  [255]

Using a more accommodating encoding:

  $ HGENCODING=UTF-8 LANGUAGE=pt_BR hg tip
  abortado: n\xc3\xa3o foi encontrado um reposit\xc3\xb3rio em '$TESTTMP' (.hg n\xc3\xa3o encontrado)! (esc)
  [255]

Different encoding:

  $ HGENCODING=Latin-1 LANGUAGE=pt_BR hg tip
  abortado: n\xe3o foi encontrado um reposit\xf3rio em '$TESTTMP' (.hg n\xe3o encontrado)! (esc)
  [255]

#endif

#if gettext

Test keyword search in translated help text:

  $ HGENCODING=UTF-8 LANGUAGE=de hg help -k Aktualisiert
  Themen:
  
   subrepos Unterarchive
  
  Befehle:
  
   pull   Ruft \xc3\x84nderungen von der angegebenen Quelle ab (esc)
   update Aktualisiert das Arbeitsverzeichnis (oder wechselt die Version)

#endif

Check Mercurial specific translation problems in each *.po files, and
tool itself by doctest

  $ cd "$TESTDIR"/../i18n
  $ python check-translation.py *.po
  $ python check-translation.py --doctest
  $ cd $TESTTMP