Show command in error message

[tiran]

Miss Islington would be even more helpful if she would just tell me the right command. For example python/cpython#3674 (review) has comment

Sorry @felipeochoa and @tiran, I had trouble checking out the 2.7 backport branch.
Please backport using cherry_picker on command line.

She has all the information to tell me the right hash and branch(es):

Sorry @felipeochoa and @tiran, I had trouble checking out the 2.7 backport branch.
Please backport using cherry_picker on command line:

cherry_picker 19e4d93 2.7``.

[Mariatta]

Great idea! Thanks. Definitely can be done :)