contrib: add showstack extension

This allows getting a Python stack trace at any time on Unix by
hitting Ctrl-\ (or Ctrl-T on BSDs). Useful for debugging mysterious
hangs on the fly. Sample output:

$ hg log -k nosuchmessage
^\
  File "/home/mpm/hg/mercurial/revset.py", line 3089, in _iterfilter
    if cond(x):
  File "/home/mpm/hg/mercurial/util.py", line 415, in f
    cache[arg] = func(arg)
  File "/home/mpm/hg/mercurial/revset.py", line 1215, in matches
    for t in c.files() + [c.user(), c.description()])
  File "/home/mpm/hg/mercurial/context.py", line 525, in files
    return self._changeset[3]
  File "/home/mpm/hg/mercurial/util.py", line 531, in __get__
    result = self.func(obj)
  File "/home/mpm/hg/mercurial/context.py", line 498, in _changeset
    return self._repo.changelog.read(self.rev())
  File "/home/mpm/hg/mercurial/changelog.py", line 338, in read
    text = self.revision(node)
  File "/home/mpm/hg/mercurial/revlog.py", line 1092, in revision
    bins = self._chunks(chain)
  File "/home/mpm/hg/mercurial/revlog.py", line 1013, in _chunks
    ladd(decompress(buffer(data, chunkstart - offset, chunklength)))
  File "/home/mpm/hg/mercurial/revlog.py", line 91, in decompress
    return _decompress(bin)
----

contrib/showstack.py

@@ -0,0 +1,17 b''
+# showstack.py - extension to dump a Python stack trace on signal
+#
+# binds to both SIGQUIT (Ctrl-\) and SIGINFO (Ctrl-T on BSDs)
+
+import sys, signal, traceback
+
+def sigshow(*args):
+    sys.stderr.write("\n")
+    traceback.print_stack(args[1], limit=10, file=sys.stderr)
+    sys.stderr.write("----\n")
+
+def extsetup(ui):
+    signal.signal(signal.SIGQUIT, sigshow)
+    try:
+        signal.signal(signal.SIGINFO, sigshow)
+    except AttributeError:
+        pass