RFR: Initial implementation of dependent pull requests

[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.

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Commit messages:
 - Initial implementation

Changes: https://git.openjdk.java.net/skara/pull/886/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=886&range=00
  Stats: 460 lines in 18 files changed: 429 ins; 12 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