Implemented `review post` command.

Highlights:
  * Rebases.
  * Checks whether user is on a story branch.
  * Prompts before posting.
  * Posts reviews in parallel.

Change-Id: d4192cc060
Story-Id: 79337172

Addressed review comments.

 * Added `git.ListBranchCommits` function. Extracted private `parseCommitLog`
 function from original `git.ListBranchCommits`.