misc: update descriptions about removed file for filectxfn

Since 650b5b6e75ed, filectxfn for memctx should return None for
removed file instead of raising IOError.

hgext/largefiles/lfcommands.py

@@ -246,7 +246,7 b' def _lfconvert_addchangeset(rsrc, rdst, '
     def getfilectx(repo, memctx, f):
         if lfutil.isstandin(f):
             # if the file isn't in the manifest then it was removed
-            # or renamed, raise IOError to indicate this
+            # or renamed, return None to indicate this
             srcfname = lfutil.splitstandin(f)
             try:
                 fctx = ctx.filectx(srcfname)

mercurial/context.py

@@ -1864,8 +1864,8 b' class memctx(committablectx):'
     commit function for every file in 'files', but calls order is
     undefined. If the file is available in the revision being
     committed (updated or added), filectxfn returns a memfilectx
-    object. If the file was removed, filectxfn raises an
-    IOError. Moved files are represented by marking the source file
+    object. If the file was removed, filectxfn return None for recent
+    Mercurial. Moved files are represented by marking the source file
     removed and the new file added with copy information (see
     memfilectx).