make hook code nicer.

pre hooks can raise exception if fail.
tidy up better if exception raised (like KeyboardInterrupt) when running
hook program.

mercurial/localrepo.py

@@ -48,7 +48,7 b' class localrepository(object):'
         except IOError:
             pass
 
-    def hook(self, name, **args):
+    def hook(self, name, throw=False, **args):
         def runhook(name, cmd):
             self.ui.note(_("running hook %s: %s\n") % (name, cmd))
             old = {}
@@ -57,21 +57,25 b' class localrepository(object):'
                 old[k] = os.environ.get(k, None)
                 os.environ[k] = v
 
+            try:
             # Hooks run in the repository root
             olddir = os.getcwd()
             os.chdir(self.root)
             r = os.system(cmd)
-            os.chdir(olddir)
-
+            finally:
             for k, v in old.items():
                 if v != None:
                     os.environ[k] = v
                 else:
                     del os.environ[k]
 
+                os.chdir(olddir)
+
             if r:
-                self.ui.warn(_("abort: %s hook failed with status %d!\n") %
-                             (name, r))
+                desc, r = util.explain_exit(r)
+                if throw:
+                    raise util.Abort(_('%s hook %s') % (name, desc))
+                self.ui.warn(_('error: %s hook %s\n') % (name, desc))
                 return False
             return True
 
@@ -372,8 +376,7 b' class localrepository(object):'
             self.ui.status(_("nothing changed\n"))
             return None
 
-        if not self.hook("precommit"):
-            return None
+        self.hook("precommit", throw=True)
 
         if not wlock:
             wlock = self.wlock()