mrdb: fix that break command cannot handle Windows paths

Before fix:
```
$ mrdb c:\tmp\b.rb
(c:\tmp\b.rb:1) break c:\tmp\b.rb:3
Class name 'c' is invalid.
```

After fix:
```
$ mrdb c:\tmp\b.rb
(c:\tmp\b.rb:1) break c:\tmp\b.rb:3
Breakpoint 1: file c:\tmp\b.rb, line 3.
```
Merge pull request #2681 from sdottaka/mrdb-break-windows

mrdb: fix that break command cannot handle Windows paths