Start preparing for 2.8.4

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/RelNotes/2.8.4.txt

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+Git v2.8.4 Release Notes
+========================
+
+Fixes since v2.8.3
+------------------
+
+ * Documentation for "git merge --verify-signatures" has been updated
+   to clarify that the signature of only the commit at the tip is
+   verified.  Also the phrasing used for signature and key validity is
+   adjusted to align with that used by OpenPGP.
+
+ * On Windows, .git and optionally any files whose name starts with a
+   dot are now marked as hidden, with a core.hideDotFiles knob to
+   customize this behaviour.
+
+ * Portability enhancement for "rebase -i" to help platforms whose
+   shell does not like "for i in <empty>" (which is not POSIX-kosher).
+
+ * "git fsck" learned to catch NUL byte in a commit object as
+   potential error and warn.
+
+ * CI test was taught to build documentation pages.
+
+ * Many 'linkgit:<git documentation page>' references were broken,
+   which are all fixed with this.
+
+Also contains other minor documentation updates and code clean-ups.