convert: improve docstrings, comments.

hgext/convert/common.py

@@ -75,7 +75,8 b' class converter_source(object):'
 
     def getfile(self, name, rev):
         """Return file contents as a string. rev is the identifier returned
-        by a previous call to getchanges().
+        by a previous call to getchanges(). Raise IOError to indicate that
+        name was deleted in rev.
         """
         raise NotImplementedError()
 
@@ -89,7 +90,7 b' class converter_source(object):'
         """Returns a tuple of (files, copies).
 
         files is a sorted list of (filename, id) tuples for all files
-        changed between version and it's first parent returned by
+        changed between version and its first parent returned by
         getcommit(). id is the source revision id of the file.
 
         copies is a dictionary of dest: source

hgext/convert/convcmd.py

@@ -79,6 +79,9 b' class converter(object):'
         self.authors = {}
         self.authorfile = None
 
+        # Record converted revisions persistently: maps source revision
+        # ID to target revision ID (both strings).  (This is how 
+        # incremental conversions work.)
         self.map = mapfile(ui, revmapfile)
 
         # Read first the dst author map if any