t7004: work around weird quoting issue with MSys

It seems to be impossible to pass the _string_ '*a*' if there is _any_
file in the current directory whose name contains the letter 'a'.  So
remove all such files for the test that needs to pass that string.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

t/t7004-tag.sh

@@ -185,8 +185,9 @@ cba
 EOF
 test_expect_success \
 	'listing tags with substring as pattern must print those matching' '
-	git-tag -l "*a*" > actual &&
-	git diff expect actual
+	rm *a* &&
+	git-tag -l "*a*" > current &&
+	git diff expect current
 '
 
 cat >expect <<EOF