tests: confirm that a badly documented extension doesn't cause a crash

An external extension whose docstring doesn't conform to Mercurial standards
used to cause crashes. Test that we omit such extensions when you do a
keyword search.

tests/test-bad-extension.t

@@ -1,40 +1,57 b''
   $ echo 'raise Exception("bit bucket overflow")' > badext.py
-  $ abspath=`pwd`/badext.py
+  $ abspathexc=`pwd`/badext.py
+
+  $ cat >baddocext.py <<EOF
+  > """
+  > baddocext is bad
+  > """
+  > EOF
+  $ abspathdoc=`pwd`/baddocext.py
 
   $ cat <<EOF >> $HGRCPATH
   > [extensions]
   > gpg =
   > hgext.gpg =
-  > badext = $abspath
+  > badext = $abspathexc
+  > baddocext = $abspathdoc
   > badext2 =
   > EOF
 
   $ hg -q help help 2>&1 |grep extension
   *** failed to import extension badext from $TESTTMP/badext.py: bit bucket overflow
   *** failed to import extension badext2: No module named badext2
 
 show traceback
 
   $ hg -q help help --traceback 2>&1 | egrep ' extension|^Exception|Traceback|ImportError'
   *** failed to import extension badext from $TESTTMP/badext.py: bit bucket overflow
   Traceback (most recent call last):
   Exception: bit bucket overflow
   *** failed to import extension badext2: No module named badext2
   Traceback (most recent call last):
   ImportError: No module named badext2
 
 show traceback for ImportError of hgext.name if debug is set
 (note that --debug option isn't applied yet when loading extensions)
 
   $ (hg -q help help --traceback --config ui.debug=True 2>&1) \
   > | grep -v '^ ' \
   > | egrep 'extension..[^p]|^Exception|Traceback|ImportError|not import'
   *** failed to import extension badext from $TESTTMP/badext.py: bit bucket overflow
   Traceback (most recent call last):
   Exception: bit bucket overflow
   could not import hgext.badext2 (No module named *badext2): trying badext2 (glob)
   Traceback (most recent call last):
   ImportError: No module named *badext2 (glob)
   *** failed to import extension badext2: No module named badext2
   Traceback (most recent call last):
   ImportError: No module named badext2
+
+confirm that there's no crash when an extension's documentation is bad
+
+  $ hg help --keyword baddocext
+  *** failed to import extension badext from $TESTTMP/badext.py: bit bucket overflow
+  *** failed to import extension badext2: No module named badext2
+  Topics:
+  
+   extensions Using Additional Features