emacs/show: fix for text/calendar display

In certain scenarios involving symlinks and setting
find-file-visit-truename, text/calendar parts were not displayed
properly.

Following a suggestion of Al Haji-Ali [1], replace the use of
get-file-buffer with find-buffer-visiting.

[1]: id:m2wmneguh8.fsf@gmail.com

emacs/notmuch-show.el

@@ -864,7 +864,7 @@ will return nil if the CID is unknown or cannot be retrieved."
 		    (unless (icalendar-import-buffer file t)
 		      (error "Icalendar import error. %s"
 			     "See *icalendar-errors* for more information"))
-		    (set-buffer (get-file-buffer file))
+		    (set-buffer (find-buffer-visiting file))
 		    (setq result (buffer-substring (point-min) (point-max)))
 		    (set-buffer-modified-p nil)
 		    (kill-buffer (current-buffer)))

test/T450-emacs-show.sh

@@ -341,7 +341,6 @@ EOF
 test_expect_equal_file EXPECTED OUTPUT
 
 test_begin_subtest "display of text/calendar, symlinked tmpdir"
-test_subtest_known_broken
 mkdir real-tmp
 ln -s real-tmp tmp
 test_emacs "(let ((notmuch-show-all-multipart/alternative-parts t)