Update draft release notes for 1.7.8.1

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

Documentation/RelNotes/1.7.8.1.txt

+Git v1.7.8.1 Release Notes
+==========================
+
+Fixes since v1.7.8.1
+--------------------
+
+ * In some codepaths (notably, checkout and merge), the ignore patterns
+   recorded in $GIT_DIR/info/exclude were not honored. They now are.
+
+ * After fetching from a remote that has very long refname, the reporting
+   output could have corrupted by overrunning a static buffer.
+
+ * "git checkout" and "git merge" treated in-tree .gitignore and exclude
+   file in $GIT_DIR/info/ directory inconsistently when deciding which
+   untracked files are ignored and expendable.
+
+Also contains minor fixes and documentation updates.

RelNotes

-Documentation/RelNotes/1.7.8.txt
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+Documentation/RelNotes/1.7.8.1.txt
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