convert: add function to test if file is from source

This adds a base implementation of a function that tests if a given file from a
target repo came from the source repo. This will be used later to detect which
files did not come from the source repo during a merge, so we can merge those
files correctly instead of dropping them.

hgext/convert/common.py

@@ -82,6 +82,13 b' class converter_source(object):'
     def after(self):
         pass
 
+    def targetfilebelongstosource(self, targetfilename):
+        """Returns true if the given targetfile belongs to the source repo. This
+        is useful when only a subdirectory of the target belongs to the source
+        repo."""
+        # For normal full repo converts, this is always True.
+        return True
+
     def setrevmap(self, revmap):
         """set the map of already-converted revisions"""
         pass

hgext/convert/convcmd.py

@@ -120,6 +120,9 b' class progresssource(object):'
                          item=file, total=self.filecount)
         return self.source.getfile(file, rev)
 
+    def targetfilebelongstosource(self, targetfilename):
+        return self.source.targetfilebelongstosource(targetfilename)
+
     def lookuprev(self, rev):
         return self.source.lookuprev(rev)