Add support for rebase

[fcollonval]

This PR

• fixes "Error Loading File Diff" after git pull --rebase #1217
• fixes diff file error in conflicted #1242

by

• adding support for rebase:
  • The status of a repository gain a state to notify if a repo is in detached head, merge, cherry-pick or rebase state
  • In case a rebase is in progress, the commit box is replaced by a group button that by default allow to continue (after resolving the conflicts) or optionally skip or abort the rebase.
• some styling improvement
• fix checking out from a detached head to another ref

[github-actions]

👈 Launch a Binder on branch fcollonval/jupyterlab-git/fix/1217-diff-rebase