fmt: handle precision 0 format stings in standard way

The C-stdlib heritage of printf/fprintf/sprintf has two odd
aspects for precisions of zero with integers. First, the zero
can be specified in any of these ways, "%4.0d", "%.0d" and
"%.d" which was not previously supported here. Secondly, the
seemingly universal interpretation of precision for integers
is that precision==0 and value==0 means print nothing at all.
The code here now handles this for integers just as the code
in big/int.c does the same for the Int type. New tests are
added to fmt_test.go to verify these changes.

R=r, r
CC=golang-dev
https://golang.org/cl/4717045

src/pkg/fmt/fmt_test.go

@@ -180,6 +180,8 @@ var fmttests = []struct {
 	{"%+d", 0, "+0"},
 	{"% d", 0, " 0"},
 	{"% d", 12345, " 12345"},
+	{"%.0d", 0, ""},
+	{"%.d", 0, ""},
 
 	// unicode format
 	{"%U", 0x1, "U+0001"},

src/pkg/fmt/format.go

@@ -166,6 +166,11 @@ func (f *fmt) fmt_boolean(v bool) {
 // integer; interprets prec but not wid.  Once formatted, result is sent to pad()
 // and then flags are cleared.
 func (f *fmt) integer(a int64, base uint64, signedness bool, digits string) {
+	// precision of 0 and value of 0 means "print nothing"
+	if f.precPresent && f.prec == 0 && a == 0 {
+		return
+	}
+
 	var buf []byte = f.intbuf[0:]
 	negative := signedness == signed && a < 0
 	if negative {

src/pkg/fmt/print.go

@@ -928,6 +928,10 @@ func (p *pp) doPrintf(format string, a []interface{}) {
 				}
 			} else {
 				p.fmt.prec, p.fmt.precPresent, i = parsenum(format, i+1, end)
+				if !p.fmt.precPresent {
+					p.fmt.prec = 0
+					p.fmt.precPresent = true
+				}
 			}
 		}
 		if i >= end {