acbs: treat /tree as a "safe directory"

[Cyanoxygen]

• This allows maintainers to build packages with correct version numbers, if their /TREE is not owned by root (e.g. using regular user for development and SSH access).
• Without this fix, ACBS will be unable to get commit, branch and working tree status (e.g. whether if it is in stable, to add extra version strings if TREE is in a topic branch) when the ABBS tree is not owned by root.

• acbs/main: install safe.directory configuration if systemd-nspawn is detected (assuming it is a Ciel container instance).
• acbs/utils: Add parameters to treat /tree as "safe directory".

[what-the-diff]

PR Summary

• New Function to Identify Container Environments
  The introduction of a function named detect_nspawn(). Its role is to determine whether the script is operating within a specific type of container environment known as systemd-nspawn.

• Establishment of a Safe Directory
  The variable GIT_CONFIG_SAFEDIR has been introduced; it holds a string of Git configuration that designates the /tree directory as a safe place to execute operations.

• Enhancements to Initialization Process
  The init() method has been revised to incorporate a safety check. This check will only write the Git config for the safe directory if the previously mentioned detect_nspawn() function indicates that the script is running in the systemd-nspawn container environment.

• Improved Git Commands for Safety
  Git commands within utils.py file have been updated to include -c safe.directory=/tree. This directive ensures that Git operations are performed safely in the previously established /tree directory.

[Cyanoxygen]

Testing

I usually use my regular user to manage ABBS tree in my Ciel workspace, therefore ACBS will always fail when getting branch and commit information. The required topic timestamp is not appended to version of the resulting package:

After the fix ACBS is able to pick up the branch and commit information, and:

The resulting package has the required timestamp appended to its version string: