Add commit:split action and rebase progress indicator

[lambdalisue]

Summary

• Add commit:split action to interactively split commits via rebase in GinLog, GinReflog, and GinStatus buffers
• Add rebase progress component and status header to show rebase state (e.g., "REBASE-i 3/10") in statusline and GinStatus buffer
• Fix buffer reload issue after bare git commands to reflect external changes

Why

Git's commit splitting workflow requires manually running several commands: starting an interactive rebase, marking a commit for editing, resetting it, staging changes incrementally, and continuing the rebase. This is tedious and error-prone for users who want to decompose a large commit into smaller, focused commits.

This PR streamlines the workflow with a single commit:split action that:

1. Automatically starts an interactive rebase on the target commit
2. Resets the commit while preserving changes in the working tree
3. Opens GinStatus so users can immediately stage and commit changes
4. Preserves user's commit.template across rebase sessions with automatic cleanup

The rebase progress indicator addresses the lack of visual feedback during rebase operations. Users can now see at a glance where they are in a rebase sequence both in the statusline (via component) and in the GinStatus buffer header.

The buffer reload fix ensures that when users run bare git commands via :Gin that modify files, those changes are immediately reflected in open buffers instead of requiring manual :checktime.

Test plan

• Test commit:split action in GinLog buffer on a commit with multiple changes
• Test commit:split action in GinReflog buffer
• Test commit:split action in GinStatus buffer (when available)
• Verify GinStatus opens after split and shows uncommitted changes
• Create multiple new commits and run git rebase --continue to verify workflow
• Verify commit.template is preserved (check with git config commit.template before/after)
• Verify cleanup occurs automatically after rebase completes
• Test rebase progress indicator in statusline with gin#component#rebase#ascii()
• Test rebase progress header appears in GinStatus during rebase
• Test with both interactive rebase (rebase -i) and non-interactive rebase
• Verify error handling when rebase is already in progress
• Test commit:split:split, commit:split:vsplit, commit:split:tabedit openers
• Run :Gin commit --amend and verify buffer reloads automatically show the change

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch add-commit-split-action

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 62.96%. Comparing base (726a2ee) to head (cb11647).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #176   +/-   ##
=======================================
  Coverage   62.96%   62.96%           
=======================================
  Files          24       24           
  Lines        1199     1199           
  Branches      172      172           
=======================================
  Hits          755      755           
  Misses        443      443           
  Partials        1        1           

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