Fix legacy URL redirects

[ethantkoenig]

Fix legacy URL redirects, which previously ignored the path component of the URL. Also adds tests.

For example, /user/repo/src/master/file.txt previously redirected to /user/repo/src/branch/master, but now redirects to /user/repo/src/branch/master/file.txt.

[codecov-io]

Codecov Report

Merging #3100 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3100      +/-   ##
==========================================
+ Coverage   34.05%   34.07%   +0.02%     
==========================================
  Files         273      273              
  Lines       39998    40002       +4     
==========================================
+ Hits        13620    13632      +12     
+ Misses      24443    24429      -14     
- Partials     1935     1941       +6

Impacted Files	Coverage Δ
modules/context/repo.go	59.88% <100%> (+0.31%)	⬆️
models/repo.go	38.15% <0%> (-0.19%)	⬇️
modules/indexer/repo.go	63.47% <0%> (+2.6%)	⬆️
models/repo_indexer.go	52.97% <0%> (+3.96%)	⬆️

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[lafriks]

LGTM

[Bwko]

LGTM

[lafriks]

@ethantkoenig can you backport this?