Add --debugger global option

With this option, hg will drop into the Python debugger on execution.
Running 'continue' will execute normally, and the debugger will be
reinvoked if an exception is raised.

mercurial/commands.py

@@ -7,7 +7,7 b''
 
 from demandload import demandload
 from node import *
-demandload(globals(), "os re sys signal shutil imp urllib")
+demandload(globals(), "os re sys signal shutil imp urllib pdb")
 demandload(globals(), "fancyopts ui hg util lock revlog")
 demandload(globals(), "fnmatch hgweb mdiff random signal time traceback")
 demandload(globals(), "errno socket version struct atexit sets bz2")
@@ -1849,6 +1849,7 b' globalopts = ['
     ('q', 'quiet', None, 'quiet mode'),
     ('v', 'verbose', None, 'verbose mode'),
     ('', 'debug', None, 'debug mode'),
+    ('', 'debugger', None, 'start debugger'),
     ('', 'traceback', None, 'print traceback on exception'),
     ('', 'time', None, 'time how long the command takes'),
     ('', 'profile', None, 'profile'),
@@ -1970,6 +1971,10 b' def dispatch(args):'
     u.updateopts(options["verbose"], options["debug"], options["quiet"],
               not options["noninteractive"])
 
+    # enter the debugger before command execution
+    if options['debugger']:
+        pdb.set_trace()
+
     try:
         try:
             if options['help']:
@@ -2011,6 +2016,9 b' def dispatch(args):'
             else:
                 return d()
         except:
+            # enter the debugger when we hit an exception
+            if options['debugger']:
+                pdb.post_mortem(sys.exc_info()[2])
             if options['traceback']:
                 traceback.print_exc()
             raise