cmd/vet: don't treat trailing % as possible formatting directive

Eliminates the following false positive:

cmd/go/go_test.go:1916: possible formatting directive in Error call

The line in question:

tg.t.Error("some coverage results are 0.0%")

Updates #11041

Change-Id: I3b7611fa3e0245714a19bd5388f21e39944f5296
Reviewed-on: https://go-review.googlesource.com/27128
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/cmd/vet/print.go

diff --git a/src/cmd/vet/print.go b/src/cmd/vet/print.go
index f4b985c..e468971 100644
--- a/src/cmd/vet/print.go
+++ b/src/cmd/vet/print.go
@@ -626,7 +626,10 @@
 	}
 	arg := args[0]
 	if lit, ok := arg.(*ast.BasicLit); ok && lit.Kind == token.STRING {
-		if strings.Contains(lit.Value, "%") {
+		// Ignore trailing % character in lit.Value.
+		// The % in "abc 0.0%" couldn't be a formatting directive.
+		s := strings.TrimSuffix(lit.Value, `%"`)
+		if strings.Contains(s, "%") {
 			f.Badf(call.Pos(), "possible formatting directive in %s call", name)
 		}
 	}