hook: add a 'hashook' function to test for hook existence

Preparing the data for some hooks can be expensive. Add a function to check if
a hook exists so we can skip useless preparation if no hook is configured.

mercurial/hook.py

@@ -189,6 +189,15 b' def redirect(state):'
     global _redirect
     _redirect = state
 
+def hashook(ui, htype):
+    """return True if a hook is configured for 'htype'"""
+    if not ui.callhooks:
+        return False
+    for hname, cmd in _allhooks(ui):
+        if hname.split('.')[0] == htype and cmd:
+            return True
+    return False
+
 def hook(ui, repo, htype, throw=False, **args):
     if not ui.callhooks:
         return False