fix: add '--' separator to Git commands to disambiguate refs from paths

[cjsoft]

Problem

When a branch name matches a filename in the working directory, Git commands like git checkout and git reset become ambiguous about whether arguments are refs or paths. This causes git-branchless commands (like git branchless amend) to fail when operating on branches that have the same name as files in the current directory.

Example scenario:

$ git checkout -b foo
$ touch foo
$ git branchless amend
# Error: pathspec 'foo' did not match any files

Solution

This PR adds the standard Git -- separator to all checkout and reset operations in the check_out module to explicitly separate refs from paths, preventing such ambiguity.

Changes

Core fix (check_out.rs): Added -- separator to 5 git command invocations:

• Line 150: git reset <target> --
• Line 159: git checkout <target> --
• Line 261: git reset --hard HEAD --
• Line 274: git checkout <oid> --
• Line 286: git reset <oid> --

Tests:

• Updated 220+ test snapshots across 15 test files to match the new command output format with -- separator
• Added new test test_amend_with_branch_name_matching_file() in test_amend.rs to verify the fix works when branch names match filenames

The -- separator is a standard Git convention that tells Git to treat everything before it as refs/options and everything after it as paths. This ensures unambiguous command parsing even when branch names and filenames collide.

Test Results

All affected test suites passing:

• ✅ test_amend (18 tests)
• ✅ test_navigation (24 tests)
• ✅ test_move (70 tests)
• ✅ test_snapshot (6 tests)
• ✅ test_split (20 tests)
• ✅ test_record (16 tests)
• ✅ test_sync, test_restack, test_reword, test_branchless
• ✅ test_github_forge, test_phabricator_forge

[cjsoft]

Core changes are in this file. Changes in other files are mainly simple replacement of test ground truth string.

[claytonrcarter]

Thank you for this! It looks like a simple enough change, and the test is self explanatory.

I'm going to approve it and then plan to merge it in a week or so, so other maintainers can weigh in if they choose to.