admin/upload_index: Reduce spawn_blocking() scope

[Turbo87]

Only the initial repository cloning and file listing operations are synchronous, but the rest of the code can live outside of spawn_blocking() these days.

repo.head_oid() is arguably also synchronous, but it is a fast enough operation to not matter too much for an admin tool :)

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 33 lines in your changes missing coverage. Please review.

Project coverage is 89.07%. Comparing base (2b82553) to head (9c44de5).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/bin/crates-admin/upload_index.rs	0.00%	33 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9919      +/-   ##
==========================================
- Coverage   89.08%   89.07%   -0.02%     
==========================================
  Files         290      290              
  Lines       30086    30088       +2     
==========================================
- Hits        26803    26801       -2     
- Misses       3283     3287       +4     

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[eth3lbert]

Sounds reasonable to me!