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>

Documentation/git-for-each-ref.txt

@@ -75,6 +75,8 @@ For all objects, the following names can be used:
 refname::
 	The name of the ref (the part after $GIT_DIR/).
 	For a non-ambiguous short name of the ref append `:short`.
+	The option core.warnAmbiguousRefs is used to select the strict
+	abbreviation mode.
 
 objecttype::
 	The type of the object (`blob`, `tree`, `commit`, `tag`).

builtin-for-each-ref.c

@@ -601,7 +601,8 @@ static void populate_value(struct refinfo *ref)
 		if (formatp) {
 			formatp++;
 			if (!strcmp(formatp, "short"))
-				refname = shorten_unambiguous_ref(refname, 0);
+				refname = shorten_unambiguous_ref(refname,
+						      warn_ambiguous_refs);
 			else
 				die("unknown %.*s format %s",
 				    (int)(formatp - name), name, formatp);
@@ -917,6 +918,9 @@ int cmd_for_each_ref(int argc, const char **argv, const char *prefix)
 		sort = default_sort();
 	sort_atom_limit = used_atom_cnt;
 
+	/* for warn_ambiguous_refs */
+	git_config(git_default_config, NULL);
+
 	memset(&cbdata, 0, sizeof(cbdata));
 	cbdata.grab_pattern = argv;
 	for_each_ref(grab_single_ref, &cbdata);

t/t6300-for-each-ref.sh

@@ -301,10 +301,11 @@ test_expect_success 'Check for invalid refname format' '
 
 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 &&
@@ -315,12 +316,23 @@ test_expect_success 'Check ambiguous head and tag refs' '
 	test_cmp expected actual
 '
 
+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 &&