When `boot2docker` is called from msys bash shell, usage string prints the
full path of the binary (e.g. `c:\program file\boot2docker for
windows\boot2docker.exe`) in the short usage string.

This change fixes that without breaking the behavior on unix systems or
cmd.exe/powershell on windows which already print the short binary name

Signed-off-by: Ahmet Alp Balkan <ahmetalpbalkan@gmail.com>

added a commit that referenced this pull request

Print only base binary name in usage string (win)

ahmetb deleted the usage-string-short branch