Allocate less on empty reads in procsrv

Author: eholk

src/compiletest/procsrv.rs

@@ -106,7 +106,7 @@ fn readclose(fd: c_int) -> ~str {
     let mut buf = ~"";
     while !reader.eof() {
         let bytes = reader.read_bytes(4096u);
-        buf += str::from_bytes(bytes);
+        str::push_str(buf, str::from_bytes(bytes));
     }
     os::fclose(file);
     return buf;