tests: fix test_commit() for case insensitive filesystems

Brian Gernhardt noticed that t3411 was broken recently on case insensitive filesystems. 0088496 (test-lib.sh: introduce test_commit() and test_merge() helpers, 2009-01-27) used a tag and a file with the same name, only different in case, and converted many existing tests that needed only a file (or a tag). Some tests may want to refer to a rev or a file, but on a filesystem that loses cases, referring to either without disambiguation mark ("--") on the command line now triggers an error (t3411 was the only one such test). Fix it by using a filename that is different from the tagname each step creates. Signed-off-by: Junio C Hamano <gitster@pobox.com>