extensions: show traceback on load failure if --traceback flag is set

Before this patch, there was no handy way to investigate the reason why
extension couldn't be loaded.

If ui.debug is set, tracebacks of both "hgext.foo" and "foo" are displayed
because the first ImportError could occur at very deep dependency module.

mercurial/extensions.py

@@ -85,6 +85,8 b' def load(ui, name, path):'
         except ImportError, err:
             ui.debug('could not import hgext.%s (%s): trying %s\n'
                      % (name, err, name))
+            if ui.debugflag:
+                ui.traceback()
             mod = importh(name)
     _extensions[shortname] = mod
     _order.append(shortname)
@@ -110,6 +112,7 b' def loadall(ui):'
             else:
                 ui.warn(_("*** failed to import extension %s: %s\n")
                         % (name, inst))
+            ui.traceback()
 
     for name in _order[newindex:]:
         uisetup = getattr(_extensions[name], 'uisetup', None)

tests/test-bad-extension.t

@@ -15,3 +15,32 b''
   hg help [-ec] [TOPIC]
   
   show help for a given topic or a help overview
+
+show traceback
+
+  $ hg -q help help --traceback 2>&1 | grep -v '^  '
+  *** 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
+  hg help [-ec] [TOPIC]
+  
+  show help for a given topic or a help overview
+
+show traceback for ImportError of hgext.name if debug is set
+(note that --debug option isn't applied yet when loading extensions)
+
+  $ hg help help --traceback --config ui.debug=True 2>&1 \
+  > | grep -v '^  ' | head -n10
+  *** 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
+  Traceback (most recent call last):
+  ImportError: No module named badext2
+  *** failed to import extension badext2: No module named badext2
+  Traceback (most recent call last):
+  ImportError: No module named badext2
+  hg help [-ec] [TOPIC]