Support for operating in plain git mode (without a multi-branch workspace)

[krlvi]

Background

GitButler currently operates on branches that are transparently integrated in an local-only octopus merge commit.

*         <- origin/main
|   m      <- gitbutler/workspace
|  / \
| a2  |    <- feature-a 
| |   b2    <- feature-b
| a1  |
|/    b1
*    /
| __/
*

In the above schematic, m is an app-managed merge commit not displayed in the app, referenced as gitbutler/workspace. Branches feature-a and feature-b are independently merge-able (i.e. not stacked) and thus can be pushed to the remote independently.

This setup facilitates the support of having multiple independent, non-stacked branches checked out at the same time.

A current limitation in GitButler is that this the merge commit m and the gitbutler/workspace branch are setup even when no multitasking is required.

*    <- origin/main
| m   <- gitbutler/workspace
| |
| a   <- feature-a 
| |
| a
|/
*

Objectives

• Support a "single branch" mode (no workspace branch, no integration merge commit)
• Switch into a workspace when it is requested from the app to apply a second concurrent branch
• Switch out of a workspace when removing a branch from the workspace leaves it with just a single branch

Sub-tasks

Foundations

• Graph based internal representation of branches that can handle the absence of the workspace merge commit (implemented in https://github.com/gitbutlerapp/gitbutler/blob/master/crates/but-graph/src/lib.rs)
• Updated commit engine that can handle the absence of the workspace merge commit (implemented in https://github.com/gitbutlerapp/gitbutler/blob/master/crates/but-workspace/src/commit_engine/mod.rs)
• New rebase engine operating on the new graph based internal data model (implemented in https://github.com/gitbutlerapp/gitbutler/blob/master/crates/but-rebase/src/lib.rs)

APIs to overhaul

• "apply to workspace" to support the absence of a workspace (implemented in https://github.com/gitbutlerapp/gitbutler/blob/master/crates/but-workspace/src/branch/apply.rs#L158-L180)
• "unapply from workspace" to support removing of the workspace (switching to single branch mode)
• "update workspace" to support updating multiple or a single branch
• "insert blank commit"
• "move changes between commits"
• "reword a commit"
• "remove changes from commit"
• "reorder a commit"
• "uncommit"
• "squash commits"