.driveignore behavior is difficult to predict

[khenriks]

I've been trying to set up drive so that when I run drive pull it will only pull the whitelisted files/directories I specify. This kind of turns the .driveignore file on its head, since it's meant to exclude rather than include. I was able to make it work thanks to the work in #535, but it doesn't seem to be fully documented, and I've had a lot of trouble making it make sense.

Eventually, I ended up modifying the regex matching function so it would print out what it was doing, and I found the following behavior:

1. The regexes listed are matched against the full path from the drive root, including a leading slash.
2. The regexes are matched against the file name by itself.
3. At the beginning, a single "/" is matched, as well as the full path to the drive root on the filesystem (e.g. /home/khenriks/myDrive).

If any of these matches indicates a file/path is excluded, then it is.

The behavior in 1 and 2 is non-obvious, but if it were documented it doesn't seem too challenging to work with. But the behavior in 3 seems very strange to me. I can't see any reason someone would want to exclude the entire drive folder, and also matching against the path on disk also doesn't seem useful. I think it would be good to skip 3 entirely.