Signed-off-by: James Couball jcouball@yahoo.com

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Description

Implements Git.default_branch(repository) which returns the default branch of a repository where repository can be a URI or a Pathname.

For instance:

require 'git'
Git.default_branch('https://github.com/ruby-git/ruby-git') #=> 'master'
Git.default_branch('https://github.com/rspec/rspec-core') #=> 'main'

Can also be used on a local repository by giving a path to the repository. In this case, it returns the branch being tracked by refs/remotes/origin/HEAD if it exists. If not, it returns the current branch.

# Prepare the current directory by:
#   git clone https://github.com/ruby-git/ruby-git test_repository
#   cd test_repository
#   # Even if the branch is changed, will still report the branch tracked by refs/remotes/origin/HEAD
#   git checkout -b new_branch
#   irb
require 'git'
Git.default_branch('.') #=> 'master'