Fix synchronization bug in repo indexer

[ethantkoenig]

Fixes a bug in the repo indexer code where local copies of repositories were accessed without the corresponding lock.

Since the repo indexer code only needs to read changes that have already been pushed, it doesn't need to access the local copies in the first place; it can just run commands against the bare repo directly.

[lafriks]

make fmt error

[ethantkoenig]

@lafriks fixed

[codecov-io]

Codecov Report

Merging #3455 into master will decrease coverage by <.01%.
The diff coverage is 64.55%.

@@            Coverage Diff             @@
##           master    #3455      +/-   ##
==========================================
- Coverage   35.68%   35.67%   -0.01%     
==========================================
  Files         281      281              
  Lines       40662    40689      +27     
==========================================
+ Hits        14509    14516       +7     
- Misses      24013    24025      +12     
- Partials     2140     2148       +8

Impacted Files	Coverage Δ
models/repo_indexer.go	47.43% <64.55%> (+0.52%)	⬆️
modules/sync/exclusive_pool.go	89.28% <0%> (-10.72%)	⬇️
models/repo.go	42.8% <0%> (-0.37%)	⬇️
models/repo_list.go	67.18% <0%> (+1.56%)	⬆️

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[lafriks]

@ethantkoenig please backport to release/v1.4 branch