Get branch name from Pull Request.

[shrimpy]

tested with below cases:

public repo from a PR:
https://localhost:44300/?repository=https://bitbucket.org/shrimpywu/publicgit?pr=1&manual=true

public repo from a branch:
https://localhost:44300/?repository=https://bitbucket.org/shrimpywu/publicgit?at=try&manual=false

private repo from a PR:
https://localhost:44300/?repository=https://bitbucket.org/shrimpywu/privatehg?pr=1&manual=false
https://localhost:44300/?repository=https://bitbucket.org/shrimpywu/privatehg?pr=1&manual=true

private repo from a branch:
https://localhost:44300/?repository=https://bitbucket.org/shrimpywu/privategit?at=TestBranch&manual=false
https://localhost:44300/?repository=https://bitbucket.org/shrimpywu/privategit?at=TestBranch&manual=true

no manual flag:
https://localhost:44300/?repository=https://bitbucket.org/shrimpywu/privategit?at=TestBranch

from github:
https://localhost:44300/?repository=https://github.com/shrimpy/gotry

[davidebbo]

:shipit: