pre-rebase hook: capture documentation in a <<here document

Without this change, the sample hook does not pass a syntax check
(sh -n):

  $ sh -n hooks--pre-rebase.sample
  hooks--pre-rebase.sample: line 101: syntax error near unexpected token `('
  hooks--pre-rebase.sample: line 101: `   merged into it again (either directly or indirectly).'

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

templates/hooks--pre-rebase.sample

@@ -88,9 +88,7 @@ else
 	exit 1
 fi
 
-exit 0
-
-################################################################
+<<\DOC_END
 
 This sample hook safeguards topic branches that have been
 published from being rewound.
@@ -167,3 +165,5 @@ To compute (2):
 	git rev-list master..topic
 
 	if this is empty, it is fully merged to "master".
+
+DOC_END