Fix regression with distutils MANIFEST handing (#11104, #8688).

The changed behavior of sdist in 2.7 broke packaging for projects that
wanted to use a manually-maintained MANIFEST file (instead of having a
MANIFEST.in template and letting distutils generate the MANIFEST).

The fixes that were committed for #8688 (d29399100973 by Tarek and
f7639dcdffc3 by me) did not fix all issues exposed in the bug report,
and also added one problem: the MANIFEST file format gained comments,
but the read_manifest method was not updated to handle (i.e. ignore)
them.  This changeset should fix everything; the tests have been
expanded and I successfully tested with Mercurial, which suffered from
this regression.

I have grouped the versionchanged directives for these bugs in one place
and added micro version numbers to help users know the quirks of the
exact version they’re using.  I also removed a stanza in the docs that
was forgotten in Tarek’s first changeset.

Initial report, thorough diagnosis and patch by John Dennis, further
work on the patch by Stephen Thorne, and a few edits and additions by
me.
diff --git a/Misc/NEWS b/Misc/NEWS
index fc4dfe9..0aeddf4 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -37,6 +37,9 @@
 Library
 -------
 
+- Issues #11104, #8688: Fix the behavior of distutils' sdist command with
+  manually-maintained MANIFEST files.
+
 - Issue #8887: "pydoc somebuiltin.somemethod" (or help('somebuiltin.somemethod')
   in Python code) now finds the doc of the method.