Improved error message for log --follow

Mads Kiilerich notes that the existing message was unclear
when a nonexistent file was supplied.

mercurial/cmdutil.py

@@ -1033,6 +1033,8 b' def walkchangerevs(ui, repo, pats, chang'
                 if node is None:
                     # A zero count may be a directory or deleted file, so
                     # try to find matching entries on the slow path.
+                    if follow:
+                        raise util.Abort(_('cannot follow nonexistent file: "%s"') % file_)
                     slowpath = True
                     break
                 else:

tests/test-log.out

@@ -5,7 +5,7 b' date: Thu Jan 01 00:00:01 1970 +0'
 summary:     a
 
 % -f, directory
-abort: can only follow copies/renames for explicit file names
+abort: cannot follow nonexistent file: "dir"
 % -f, but no args
 changeset:   4:b30c444c7c84
 tag:         tip