fmt: measure width in runes not bytes with %c and %q for ints

This is meant to share my progress on Issue 8275, if it's useful to you. I'm not familiar with the formatter's internals, so this change is likely naive.

Change these calls to measure width in runes not bytes:
fmt.Printf("(%5q)\n", '§')
fmt.Printf("(%3c)\n", '§')

 Fixes #8275.

LGTM=r
R=golang-codereviews, r
CC=golang-codereviews
https://golang.org/cl/104320043

src/pkg/fmt/fmt_test.go

@@ -183,6 +183,8 @@ var fmtTests = []struct {
 	{"%.3q", "日本語日本語", `"日本語"`},
 	{"%.3q", []byte("日本語日本語"), `"日本語"`},
 	{"%10.1q", "日本語日本語", `       "日"`},
+	{"%3c", '⌘', "  ⌘"},
+	{"%5q", '\u2026', `  '…'`},
 	{"%10v", nil, "     <nil>"},
 	{"%-10v", nil, "<nil>     "},
 

src/pkg/fmt/format.go

@@ -114,7 +114,7 @@ func (f *fmt) pad(b []byte) {
 		f.buf.Write(b)
 		return
 	}
-	padding, left, right := f.computePadding(len(b))
+	padding, left, right := f.computePadding(utf8.RuneCount(b))
 	if left > 0 {
 		f.writePadding(left, padding)
 	}