RFR: Initial implementation of dependent pull requests [v2]

[Robin Westberg]

> This is the initial implementation of the support for creating a follow-up pull request while the first one is still
> out for review. This will allow having multiple dependent changes out for review simultaneously, which is something
> that was possible during the hg/webrev era.  To use it, first create a pull request from the first change. In that pull
> request, issue the `/preintegrate` command. This will publish the current contents to a new branch in the *target*
> repository named `pr/<username>/<pr-id>`. Now any follow-up work can target this branch instead, which contains the
> dependent work.  When the first pull request is `/integrate`d, any follow-up pull requests will automatically be
> retargeted to the original target branch. It will not be possible to `/integrate` the follow-up pull request until the
> first one has been integrated.

Robin Westberg has updated the pull request incrementally with one additional commit since the last revision:

  Update pre-integration target branches automatically

-------------

Changes:
  - all: https://git.openjdk.java.net/skara/pull/886/files
  - new: https://git.openjdk.java.net/skara/pull/886/files/24270bf0..45ca450e

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=skara&pr=886&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=skara&pr=886&range=00-01

  Stats: 492 lines in 13 files changed: 336 ins; 137 del; 19 mod
  Patch: https://git.openjdk.java.net/skara/pull/886.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/886/head:pull/886

PR: https://git.openjdk.java.net/skara/pull/886