hooks: forbid ':' in hook name

The `:` character is a special separator in the config and it seems same do to
the same for hooks. This is necessary to improve the experience around the
HGPLAIN behavior change in 5.7. See next changesets for details.

Differential Revision: https://phab.mercurial-scm.org/D9978

mercurial/configitems.py

@@ -1310,7 +1310,7 b' coreconfigitem('
 )
 coreconfigitem(
     b'hooks',
-    b'.*',
+    b'[^:]*',
     default=dynamicdefault,
     generic=True,
 )

mercurial/hook.py

@@ -224,7 +224,11 b' def _hookitems(ui, _untrusted=False):'
     """return all hooks items ready to be sorted"""
     hooks = {}
     for name, cmd in ui.configitems(b'hooks', untrusted=_untrusted):
-        if name.startswith(b'priority.') or name.startswith(b'tonative.'):
+        if (
+            name.startswith(b'priority.')
+            or name.startswith(b'tonative.')
+            or b':' in name
+        ):
             continue
 
         priority = ui.configint(b'hooks', b'priority.%s' % name, 0)