bin/git-post-land: replace confirmation with undo
bin/git-post-land: replace confirmation with undo

It is generally[1] considered better usability to
support an undo option, when possible, rather than a
confirmation step (aka a warning).

[1]: http://alistapart.com/article/neveruseawarning

Closes chain/chainprv#1668
Reviewers: da39a3ee5e6b4b0d3255bfef95601890afd80709@erykwalder

chain/chain,chain/chain,chain/chain,chain/chain,chain/chain,chain/chain