ycmd: improve search of nearest file in compilation database

[deb0ch]

The old version wasn't thorough enough, as it only searched subdirectories recursively with os.walk().

This version will search current directory recursively with os.walk() and then do the same recursively for parent directories until a match is found or we searched the database directory.

[TheBB]

Thanks! Cherry-picked in develop. You can safely delete your branch.