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/image_test.go

@@ -27,7 +27,7 @@ func TestImageContext(t *testing.T) {
 	}{
 		{
 			imageCtx: imageContext{i: image.Summary{ID: imageID}, trunc: true},
-			expValue: stringid.TruncateID(imageID),
+			expValue: TruncateID(imageID),
 			call:     ctx.ID,
 		},
 		{