tests/test-diff-copy-depth.out

add documentation for revlog._prereadsize

-- With aaa
Status:
A somename
  other-file

Diff:
diff --git a/other-file b/somename
copy from other-file
copy to somename

-- With zzz
Status:
A somename
  other-file

Diff:
diff --git a/other-file b/somename
copy from other-file
copy to somename