Provide fast path in snprintf.c for conversion specs that are just "%s".

This case occurs often enough (around 45% of conversion specs executed
in our regression tests are just "%s") that it's worth an extra test
per conversion spec to allow skipping all the logic associated with
field widths and padding when it happens.

Discussion: https://postgr.es/m/26193.1538582367@sss.pgh.pa.us

src/port/snprintf.c

@@ -431,6 +431,19 @@ dopr(PrintfTarget *target, const char *format, va_list args)
 
 		/* Process conversion spec starting at *format */
 		format++;
+
+		/* Fast path for conversion spec that is exactly %s */
+		if (*format == 's')
+		{
+			format++;
+			strvalue = va_arg(args, char *);
+			Assert(strvalue != NULL);
+			dostr(strvalue, strlen(strvalue), target);
+			if (target->failed)
+				break;
+			continue;
+		}
+
 		fieldwidth = precision = zpad = leftjust = forcesign = 0;
 		longflag = longlongflag = pointflag = 0;
 		fmtpos = accum = 0;