Fix Copier conflict detection for whitespace and new files

[timkpaine]

Copier updates such as tkp-aliases/nbprint-logfire#33 received conflict warnings because git diff --check also reports whitespace errors. That PR added four Makefile lines with trailing spaces and contained no conflict markers.

Disable whitespace diagnostics for the conflict check and inspect staged changes after lockfile refresh, so newly added files are checked too. Warn about unresolved conflict markers explicitly, and propagate other Git failures.

make lint checks coverage passes: 26 tests. Real Git regression cases reproduce the previous whitespace false positives and missed conflicts in new files, then pass with this fix.

[github-actions]

Test Results

26 tests  +8   26 ✅ +8   1s ⏱️ -1s
 1 suites ±0    0 💤 ±0 
 1 files   ±0    0 ❌ ±0 

Results for commit 9f4ef98. ± Comparison against base commit bb6435c.

[codecov]

Codecov Report

❌ Patch coverage is 95.34884% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.46%. Comparing base (bb6435c) to head (9f4ef98).

Files with missing lines	Patch %	Lines
copier_update/updater.py	75.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #24      +/-   ##
==========================================
+ Coverage   84.69%   85.46%   +0.76%     
==========================================
  Files           8        8              
  Lines         477      509      +32     
  Branches       41       46       +5     
==========================================
+ Hits          404      435      +31     
- Misses         58       59       +1     
  Partials       15       15              

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