Add new multi-git script: `git-activity-report`

[smondet]

• This is still work in progress.
• The tests do not verify much yet either.

[smondet]

Example:

git activity-report --since 2018-10-21

GHub: genspio

*   HEAD -> sm@add-activity-report, origin/sm@add-activity-report
|\  
| * origin/master, origin/HEAD, master
* | refs/pull/origin/98
|/  
* origin/sm@add-multigit-example, refs/pull/origin/97, sm@add-multigit-example

On master:

• Tweak vm-tester documentation w.r.t Docker images.
• Add docker-build trigger in Travis build.
• Trigger docker-build only in one travis-job. (and fix it).
• Add the “Multi-git” example. For now there is a git multi-status <paths> command.
• Fix untracked count in multi_status.
• Add multigit to TravisCI.
• Put multigit test in separate script.
• Fix multi-status tests in OSX.
• Add option --show-modified (git multi-status).
• Display repository provider (git multi-status).
• Put descriptive comments in git-multi-status.
• Add multigit to the literally documented examples.
• Add support for git-config and improve help.
• Merge sm@add-multigit-example (PR #97).

On sm@add-activity-report

• Add new multi-git script: git-activity-report.
• This is still work in progress.
• The tests do not verify much yet either.
• Merge master into sm@add-activity-report.

GLab: genspio-doc

* HEAD -> master, origin/master, origin/HEAD

On master:

• Update .gitlab-ci.yml (missing in fb3bbbd).

[smondet]

This is merged...