fix(git): handle submodule gitlinks and directory symlinks in working-copy snapshots

Cherry-picked from my fork's megamerge branch.

Fix panics in working-copy snapshot/amend paths when changed entries are not
regular files.

Previously, snapshot/amend attempted to create blobs by reading paths from
disk unconditionally. That fails for:
- submodule gitlinks (mode 160000), where path is a directory in superproject
- directory symlinks like `ln -s dir/ link`, where symlink handling must use
  link target bytes, not file read semantics

Changes:
- add mode-aware object creation for working-copy paths
  - regular blobs: read file bytes
  - symlinks: read symlink target and store as blob content
  - gitlinks/submodules: use index OID instead of reading filesystem path
  - unreadable/tree: skip as absent
- wire mode-aware logic into:
  - snapshot unstaged commit creation
  - amend_fast working-copy path processing
- add regressions:
  - dirty submodule no longer crashes status/snapshot flow
  - trailing-slash directory symlink no longer crashes snapshot flow
  - amend with dirty submodule no longer crashes
- make tests use per-repo submodule source paths to avoid cross-test collisions

Fixes https://github.com/arxanas/git-branchless/issues/517
Fixes https://github.com/arxanas/git-branchless/issues/645

Co-authored-by: pi with GPT-5.3-Codex <pi-gpt-5.3-codex@codex.local>