feat(updating): propagate exec bit on new template files when core.fileMode=false

[willemkokke]

Summary

Closes #2630. Follow-up to #2605.

#2605 fixed executable-bit propagation for files that already exist in the destination. Files introduced by the template for the first time on copier update were still affected when core.fileMode=false:

1. Template v2 adds a new script.sh with 100755 in the index.
2. Destination has core.fileMode=false (Windows default).
3. copier writes the file, chmods it on disk (invisible to git with fileMode=false), then calls _sync_git_index_executable_bit — which short-circuited because the file is untracked.
4. The user's next git add script.sh records 100644 because git ignores on-disk mode with fileMode=false. Exec bit lost.

Changes

copier/_main.py — _sync_git_index_executable_bit:

For untracked files that should be executable, run git add --intent-to-add <path> to register an empty index entry, then fall through to the existing --cacheinfo path to set the mode to 100755. The user's eventual git add replaces the empty blob with the file's actual content while preserving the mode bit. The intent-to-add marker (A in git status) is also a more accurate signal than ?? for files that are deliberate template output.

• Skipped when the file does not need an executable bit (the user's git add would record 100644 either way).
• Skipped when core.fileMode=true (git picks up the on-disk chmod naturally — pre-staging would be inconsistent with copier's "leave changes unstaged for user review" behavior).

tests/test_updatediff.py — 3 new tests:

Test	What it covers
test_update_introduces_new_executable_file[True|False]	New +x file in template v2: index records 100755 on fileMode=false; file stays untracked on fileMode=true
test_update_introduces_new_non_executable_file[True|False]	New non-exec file is NOT pre-staged in either fileMode case
test_update_introduces_new_executable_file_then_user_commits	End-to-end on core.fileMode=false: copier update → git add → git commit → committed tree records 100755 and the file's actual content. The template setup uses git update-index --chmod=+x (rather than Path.chmod) so the test runs identically on every platform — Linux, macOS, and Windows.

Test plan

• All new tests pass on macOS and windows
• Full test suite still green on macOS (1073 passed)
• ruff check, ruff format, mypy, editorconfig-checker all clean

[codecov]

Codecov Report

❌ Patch coverage is 96.47059% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.10%. Comparing base (e05ddfe) to head (88beeaa).

Files with missing lines	Patch %	Lines
copier/_main.py	66.66%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2640      +/-   ##
==========================================
- Coverage   97.15%   97.10%   -0.06%     
==========================================
  Files          60       60              
  Lines        7101     7185      +84     
==========================================
+ Hits         6899     6977      +78     
- Misses        202      208       +6     

Flag	Coverage Δ
unittests	97.10% <96.47%> (-0.06%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.