gps: Return output buffer bytes on error

This is crude, but it will restore output context to various git errors,
which has been crippling. We desperately need to work on formatting
these errors better.

gps/cmd_unix.go

@@ -77,8 +77,6 @@ func (c cmd) CombinedOutput() ([]byte, error) {
 		}
 	}()
 
-	if err := c.Cmd.Wait(); err != nil {
-		return nil, err
-	}
-	return b.Bytes(), nil
+	err := c.Cmd.Wait()
+	return b.Bytes(), err
 }