cli/command/formatter: add TrunateID utility

We were depending on pkg/stringid to truncate IDs for presentation. While
traditionally, we used a fixed length for "truncated" IDs, this is not
a strict requirement (any ID-prefix should work, but conflicts may
happen on shorter IDs).

This patch adds a local `TruncateID()` utility in the formatter package;
it's currently using the same implementation and length as the
`stringid.TruncateID` function, but may diverge in future.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/command/formatter/displayutils_test.go

@@ -7,6 +7,49 @@ import (
 	is "gotest.tools/v3/assert/cmp"
 )
 
+func TestTruncateID(t *testing.T) {
+	tests := []struct {
+		doc, id, expected string
+	}{
+		{
+			doc:      "empty ID",
+			id:       "",
+			expected: "",
+		},
+		{
+			// IDs are expected to be 12 (short) or 64 characters, and not be numeric only,
+			// but TruncateID should handle these gracefully.
+			doc:      "invalid ID",
+			id:       "1234",
+			expected: "1234",
+		},
+		{
+			doc:      "full ID",
+			id:       "90435eec5c4e124e741ef731e118be2fc799a68aba0466ec17717f24ce2ae6a2",
+			expected: "90435eec5c4e",
+		},
+		{
+			doc:      "digest",
+			id:       "sha256:90435eec5c4e124e741ef731e118be2fc799a68aba0466ec17717f24ce2ae6a2",
+			expected: "90435eec5c4e",
+		},
+		{
+			doc:      "very long ID",
+			id:       "90435eec5c4e124e741ef731e118be2fc799a68aba0466ec17717f24ce2ae6a290435eec5c4e124e741ef731e118be2fc799a68aba0466ec17717f24ce2ae6a2",
+			expected: "90435eec5c4e",
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.doc, func(t *testing.T) {
+			actual := TruncateID(tc.id)
+			if actual != tc.expected {
+				t.Errorf("expected: %q, got: %q", tc.expected, actual)
+			}
+		})
+	}
+}
+
 func TestEllipsis(t *testing.T) {
 	testcases := []struct {
 		source   string