filemerge: add a before-merge callback to internal merge tools

We're going to separate the pre-merge and merge steps for merge tools. The
merge step will be specific to the tool, but the pre-merge step will be common
to all merge tools that need it.

However, some merge tools run checks *before* the pre-merge step. This callback
will allow that to continue to work.

mercurial/filemerge.py

@@ -38,7 +38,7 b' internals = {}'
 # Merge tools to document.
 internalsdoc = {}
 
-def internaltool(name, trymerge, onfailure=None):
+def internaltool(name, trymerge, onfailure=None, precheck=None):
     '''return a decorator for populating internal merge tool table'''
     def decorator(func):
         fullname = ':' + name
@@ -48,6 +48,7 b' def internaltool(name, trymerge, onfailu'
         internalsdoc[fullname] = func
         func.trymerge = trymerge
         func.onfailure = onfailure
+        func.precheck = precheck
         return func
     return decorator