ssh/terminal: fix SetSize when nothing on current line

SetSize has a problem may cause the following ReadPassword setting
temporary prompt not working, when changing width the current
SetSize will call clearAndRepaintLinePlusNPrevious which would
print an old prompt whatever the current line has, causing a following
ReadPassword with temporary prompt not printing the different prompt.

When running code like this, the nt.SetSize prints a "> " as
prompt then the temporary "Password: " prompt would never show up.

```go
        oldState, err := terminal.MakeRaw(int(os.Stdin.Fd()))
        width, height, _ = terminal.GetSize(int(os.Stdin.Fd()))
        nt := terminal.NewTerminal(os.Stdin, "> ")
        nt.SetSize(width, height)
        password, err = nt.ReadPassword("Password: ")
```

the new test cases is to test SetSize with different terminal sizes,
either shrinking or expanding, a following ReadPassword should get the
correct temporary prompt.

Change-Id: I33d13b2c732997c0c88670d53545b8c0048b94b6
Reviewed-on: https://go-review.googlesource.com/1861
Reviewed-by: Adam Langley <agl@golang.org>