Fix default branch extraction

[colinodell]

The regular expression used to extract default branch names failed to match dash and period characters. As a result, get_default_branch() would return incorrect results. For example:

Actual Branch Name	Returned String
main	main
2.x	2 <-- incorrect
my-default-branch	my <-- incorrect
my_default_branch	my_default_branch

This PR fixes this issue.

To test this, we also make a slight tweak to the _initialize_multiple_empty_repos() helper to set the default branch when the repository is first created. This requires using the -b / --initial-branch option added in git 2.28. The devbox Docker image was using an older version of git (2.20), so that has been updated to 2.30 via buster-backports, ensuring the tests pass both locally (via devbox) and via GitHub Actions.

[codecov]

Codecov Report

Merging #250 (0dad388) into main (68fa58d) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #250   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines          164       164           
  Branches        22        22           
=========================================
  Hits           164       164           

Impacted Files	Coverage Δ
pygitops/operations.py	100.00% <100.00%> (ø)