Trying to fix #1069

https://github.com/Valloric/YouCompleteMe/issues/1069

To reproduce:
1. Create any file with unicode character on its filename, e.g Ω.c
2. Open the file and start adding codes until >5 lines (i.e ycm start compiling)
3. YCM will crash due to unicode encoding error. See issue referenced above for complete log.

Possible cause : On the function `GetBufferNumberForFilename` ,trying to format non-unicode string using unicode string
Workaround : Convert the full path to utf-8 before passing to bufnr
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