for-each-ref: utilize core.warnAmbiguousRefs for :short-format

core.warnAmbiguousRefs is used to select strict mode for the
abbreviation for the ":short" format specifier of "refname" and "upstream".

In strict mode, the abbreviated ref will never trigger the
'warn_ambiguous_refs' warning. I.e. for these refs:

  refs/heads/xyzzy
  refs/tags/xyzzy

the abbreviated forms are:

  heads/xyzzy
  tags/xyzzy

Signed-off-by: Bert Wesarg <bert.wesarg@googlemail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/t6300-for-each-ref.sh b/t/t6300-for-each-ref.sh
index daf02d5..8052c86 100755
--- a/t/t6300-for-each-ref.sh
+++ b/t/t6300-for-each-ref.sh
@@ -301,10 +301,11 @@
 
 cat >expected <<\EOF
 heads/master
-master
+tags/master
 EOF
 
-test_expect_success 'Check ambiguous head and tag refs' '
+test_expect_success 'Check ambiguous head and tag refs (strict)' '
+	git config --bool core.warnambiguousrefs true &&
 	git checkout -b newtag &&
 	echo "Using $datestamp" > one &&
 	git add one &&
@@ -316,11 +317,22 @@
 '
 
 cat >expected <<\EOF
+heads/master
+master
+EOF
+
+test_expect_success 'Check ambiguous head and tag refs (loose)' '
+	git config --bool core.warnambiguousrefs false &&
+	git for-each-ref --format "%(refname:short)" refs/heads/master refs/tags/master >actual &&
+	test_cmp expected actual
+'
+
+cat >expected <<\EOF
 heads/ambiguous
 ambiguous
 EOF
 
-test_expect_success 'Check ambiguous head and tag refs II' '
+test_expect_success 'Check ambiguous head and tag refs II (loose)' '
 	git checkout master &&
 	git tag ambiguous testtag^0 &&
 	git branch ambiguous testtag^0 &&