subrepo: chain the original exception to SubrepoAbort

The tracebacks in subrepos are truncated at the point where the original
exception is caught and SubrepoAbort is raised in its place since 9e3910db4e78.
That hides the most relevant subrepo methods when an error occurs.  Python 2.x
doesn't support chaining exceptions, so it is manually done here for manual
printing later.

mercurial/subrepo.py

@@ -5,7 +5,7 b''
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 
-import errno, os, re, xml.dom.minidom, shutil, posixpath
+import errno, os, re, xml.dom.minidom, shutil, posixpath, sys
 import stat, subprocess, tarfile
 from i18n import _
 import config, scmutil, util, node, error, cmdutil, bookmarks, match as matchmod
@@ -41,6 +41,7 b' class SubrepoAbort(error.Abort):'
     def __init__(self, *args, **kw):
         error.Abort.__init__(self, *args, **kw)
         self.subrepo = kw.get('subrepo')
+        self.cause = kw.get('cause')
 
 def annotatesubrepoerror(func):
     def decoratedmethod(self, *args, **kargs):
@@ -53,7 +54,8 b' def annotatesubrepoerror(func):'
             subrepo = subrelpath(self)
             errormsg = str(ex) + ' ' + _('(in subrepo %s)') % subrepo
             # avoid handling this exception by raising a SubrepoAbort exception
-            raise SubrepoAbort(errormsg, hint=ex.hint, subrepo=subrepo)
+            raise SubrepoAbort(errormsg, hint=ex.hint, subrepo=subrepo,
+                               cause=sys.exc_info())
         return res
     return decoratedmethod