Pull request page should provide hg commands for manual merge (BB-7579)

Currently when user want to somehow "test" pull request code before approving it it must manually find 2 things:

1. branch/commit id (switch from diff to commits tab, select commit id which is link and it's far from accessible)
2. url to "remote" repo to be abble to pull changeset from...

Same thing happens when pull request merge encounter "unknown error" issue.

Pull requests that have conflicts provide far more usable copy-paste info than "good" & clean ones...

To put things even further even clean pull requests have necessary code line HIDDEN in invisible div:

#!html

<div class="aui-message warning conflicts-detected">
[cut...]
<span class="command">hg pull -r 29a03fb79962 ssh://hg@bitbucket.org/user/repo</span>

Please provide button that would show this code when user needs it, like in merge buttons line: [ Merge | Manual | Edit | Decline ]