I attach a patch for this bug.

It makes dired-mark-unmarked-files act case-insensitively. This seems
reasonable, as there's nothing to suggest it does otherwise.

Also, it is better than Stefan's suggestion to use auto-mode-case-fold, as
when visiting files, the user will quickly see if a file has been e.g.
visited in the wrong major mode owing to an uppercase filename being
treated as a lowercase one; whereas in dired-omit-mode, files can be hidden
counter to the user's expectation, as happened to me with COPYING.LIB, yet
it is not obvious (because the file is hidden!).

Is this OK to install?

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