Declare that the next one will be named 2.20

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

Documentation/RelNotes/2.20.0.txt

diff --git a/RelNotes b/Documentation/RelNotes/2.20.0.txt
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rename from RelNotes
rename to Documentation/RelNotes/2.20.0.txt

RelNotes

diff --git a/RelNotes b/RelNotes
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+Documentation/RelNotes/2.20.0.txt
\ No newline at end of file