Validation fix, GitError exception instantiation fixes

[renatav]

Description (e.g. "Related to ...", etc.)

• When only validating repositories, validation would fail if there were no local branches. Commits can still be listed given a remote branch. Local branches do not exist after a repository is cloned unless it is checked out.
• Fix GitError exception instantiation
• Update find_first_branch_matching_pattern return - only return name of the found branch and not all branches whose name did not match the pattern

Code review checklist (for code reviewer to complete)

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• Commit messages are meaningful (see this for details)
• Tests have been included and/or updated, as appropriate
• Docstrings have been included and/or updated, as appropriate
• Changelog has been updated, as needed (see CHANGELOG.md)