GNU bug report logs - #29855
mention that diff -s shows more than just identical files

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From: =?utf-8?B?56mN5Li55bC8?= Dan Jacobson <jidanni@HIDDEN>
To: bug-diffutils@HIDDEN
Subject: mention that diff -s shows more than just identical files
Date: Tue, 26 Dec 2017 19:40:53 +0800
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In (info "(diffutils) Comparing Directories")

"Normally 'diff' is silent about pairs of files that contain
no differences, but if you use the '--report-identical-files' ('-s')
option, it reports pairs of identical files."

OK, but add: To see ONLY the identical files, use
$ diff --recursive --brief --report-identical-files tree1 tree2 | grep identical$