Alibuild

--fetch-repos` introduced in v1.5.1 and updated with a first round of fixes in v1.5.2 had an additional problem: local branches were not updated according to the remote counterparts in case fast-forwarding wasn't possible. If force-updates are not explicitly enabled, the `git fetch` command as it was formulated exited with a non-zero exit code. This update should now fix for good the update behaviour.

The update behaviour introduced in v1.5.1 caused `--fetch-repos` not to always fetch the updates for all the remote branches. By default it worked just for the default remote branch of a given repository. This patch release fixes the critical problem and makes `--fetch-repos` behave as expected.

Bug fixes:

* `alienv` works properly with the latest version of `environment-modules` (4.1.0): the problem was mainly affecting macOS users installing the package with Homebrew
* On some configurations, `aliBuild` did not determine whether to pick a system package or not. This is why the check script wasn't run with `bash` though we use bashisms in our recipe. We now force system checks to be always performed with `bash`

Improvements:

* Remote repository fetches are no longer performed by default, resulting in zero "fetch errors" and a much faster `aliBuild` startup time. This is useful when rebuilding frequently
* Fedora has been added to the list of supported platforms
* In non-debug mode, we now show the build percentage: it is guessed from stdout and useful for CMake-based projects

We now force environment-modules to run in 3.x compatible mode to prevent further breaks. In particular, a critical problem with the new `modulecmd` aborting when a dependency is not found was fixed (as our GCC-Toolchain selection mechanism is based on that, for instance).

* Read-only `MIRROR` directory is now handled correctly
* Added proper documentation and troubleshooting
* General optimization of tests related to this feature

Bug fixes:

* Checking for system deps resulted in inconsistent exit codes: force use of /bin/bash when checking system deps
* Running aliBuild was slow and randomly returned errors at the initial fetching phase: do it only when needed, with a switch to force-enable it
* Homebrew-provided environment-modules is now at version 4, breaking alienv: alienv now supports it fully

General improvements:

* Add Fedora to the list of supported platforms
* Show build percentage in non-debug mode (guess it from stdout, useful for CMake-based projects)