add support for --sync-pr-with-develop

[boegel]

With --sync-pr-with-develop, you can sync the branch (on GitHub) that corresponds to a specific pull request with the latest version of the develop branch from the central repository.

Examples:

• syncing {bio}[GCCcore/10.20] ack v3.4.0 w/ Perl 5.32.0 easybuild-easyconfigs#7277 with develop branch from central repository (https://github.com/easybuilders/easybuild-easyconfigs):

  $ eb --sync-pr-with-develop 7277
  == temporary log file in case of crash /tmp/eb-Uul_00/easybuild-kwe1IC.log
  == Determined branch name corresponding to easybuilders/easybuild-easyconfigs PR #7277: 20181210212203_new_pr_ack224
  == cloning git repo from /user/example/easybuild-easyconfigs...
  == fetching branch '20181210212203_new_pr_ack224' from https://github.com/boegel/easybuild-easyconfigs.git...
  == pulling latest version of 'easybuilders' branch from easybuild-easyconfigs/develop...
  == merging 'develop' branch into PR branch '20181210212203_new_pr_ack224'...
  == pushing branch '20181210212203_new_pr_ack224' to remote 'github_boegel_IhzHm' (git@github.com:boegel/easybuild-easyconfigs.git)
  == Temporary log file(s) /tmp/eb-Uul_00/easybuild-kwe1IC.log* have been removed.
  == Temporary directory /tmp/eb-Uul_00 has been removed.
  

  resulting commit: easybuilders/easybuild-easyconfigs@c086627

• dry run of syncing make bootstrap script compatible with Python 3 (WIP) #3028 with develop branch from central repository (https://github.com/easybuilders/easybuild-framework):

  $ eb --sync-pr 3028 --pr-target-repo easybuild-framework --dry-run
  == temporary log file in case of crash /tmp/eb-NLTcIF/easybuild-zDtzwv.log
  == Determined branch name corresponding to easybuilders/easybuild-framework PR #3028: bootstrap_py3
  == cloning git repo from /home/example/easybuild-framework...
  == fetching branch 'bootstrap_py3' from https://github.com/boegel/easybuild-framework.git...
  == pulling latest version of 'easybuilders' branch from easybuild-framework/develop...
  == merging 'develop' branch into PR branch 'bootstrap_py3'...
  == pushing branch 'bootstrap_py3' to remote 'github_boegel_SRxxW' (git@github.com:boegel/easybuild-framework.git) [DRY RUN]
  == Temporary log file(s) /tmp/eb-NLTcIF/easybuild-zDtzwv.log* have been removed.
  == Temporary directory /tmp/eb-NLTcIF has been removed.
  

  (same result with -D or -x instead of --dry-run)

Just using --sync-pr works too, since there's only one option that starts with --sync-pr...

[migueldiascosta]

lgtm

[migueldiascosta]

Going in, thanks @boegel!