kbfs filesystem not able to copy files with square brackets in name (need to be escaped)

files with [ or ] in name need to be escaped for ls, cp, mv, but not read

It's impossible to use the results of `keybase fs ls` as arguments for `keybase fs cp` for example.

Apparently the problem only occurs in the source argument and not the destination.

The following session dump should be self-explanatory.

```
$ cat > file.txt
123
Test
321
$ keybase fs mkdir /keybase/public/user/test
$ keybase fs cp file.txt /keybase/public/user/test/
$ keybase fs ls /keybase/public/user/test/
file.txt
$ keybase fs read /keybase/public/user/test/file.txt
123
Test
321
$ keybase fs cp /keybase/public/user/test/file.txt '/keybase/public/user/test/file [test].txt'
$ keybase fs read '/keybase/public/user/test/file [test].txt'
123
Test
321
$ keybase fs ls /keybase/public/user/test/
file [test].txt file.txt
$ keybase fs cp '/keybase/public/user/test/file [test].txt' '/keybase/public/user/test/file [test 2].txt'
▶ ERROR Async result not found
$ keybase fs read '/keybase/public/user/test/file [test].txt'
123
Test
321
$ keybase fs cp '/keybase/public/user/test/file [test].txt' '/keybase/public/user/''
▶ ERROR Async result not found
$ keybase fs cp '/keybase/public/user/test/file \[test\].txt' '/keybase/public/user/'
$ keybase fs read '/keybase/public/user/file [test].txt'
123
Test
321
```

Start with the `keybase fs cp` command and reproduce the documented session using source paths containing square brackets. Trace how source arguments are parsed and compare that behavior with `keybase fs read`; done means copying bracket-containing source paths works without manual escaping while destination paths continue to work.

Written by the indexing model from the issue text.