Fix help for file_collision method without block

[shuuuuun]

Fix #818

Branch help for file_collision depending on block_given.

This will change the order of the help slightly, but probably not a problem.

When I ran the sample code described in the issue(#818), it looked like this:

$ ruby ./thor.rb hello
=== file_collision with no block
Overwrite ./foo.txt? (enter "h" for help) [Ynaqh] h
        Y - yes, overwrite
        n - no, do not overwrite
        a - all, overwrite this and all others
        q - quit, abort
        h - help, show this help
Overwrite ./foo.txt? (enter "h" for help) [Ynaqh] y
=== file_collision with block
Overwrite ./foo.txt? (enter "h" for help) [Ynaqdhm] h
        Y - yes, overwrite
        n - no, do not overwrite
        a - all, overwrite this and all others
        q - quit, abort
        h - help, show this help
        d - diff, show the differences between the old and the new
        m - merge, run merge tool
Overwrite ./foo.txt? (enter "h" for help) [Ynaqdhm] d
- file_collision with no block
+ file_collision with block
Retrying...
Overwrite ./foo.txt? (enter "h" for help) [Ynaqdhm] y