"pr show" let you choose one pull request if the current branch has m…

[takuji]

This pull request changes the behavior of "gitb pr show" command. In case where the current branch has multiple pull requests, the command shows the options which pull request to show.

For example, it shows the pull request selector like this:

Multiple pull requests found. Select one:

> TEST/example #123
  TEST/example #120

Press 'q' to quit, 'enter' to select.

You move the cursor using ↑, ↓, j, k keys, then press the enter key to open the selected pull request page on the web browser.