Add orphaned git command topics to global nav

This MR adds several orphaned git command topics to the global nav menu.

• https://docs.gitlab.com/ee/topics/git/bisect.html

• https://docs.gitlab.com/ee/topics/git/feature_branching.html

• https://docs.gitlab.com/ee/topics/git/git_add.html

• https://docs.gitlab.com/ee/topics/git/git_log.html

• https://docs.gitlab.com/ee/topics/git/merge_conflicts.html

• https://docs.gitlab.com/ee/topics/git/rollback_commits.html

• https://docs.gitlab.com/ee/topics/git/stash.html

• https://docs.gitlab.com/ee/topics/git/subtree.html

• https://docs.gitlab.com/ee/topics/git/unstage.html

• Link to the merge request of the new page:

• Give the MR a descriptive title.

• Link to the merge request that introduced the new page.