Remove seen map from getLastCommitForPaths

[filipnavara]

Ensures correctly traversing the commit graph for all path and avoids
erroneously skipping some. Also preallocate some arrays to correct size
to prevent unnecessary reallocations.

Fixes #6708.

[filipnavara]

Note that on specially corrupted repositories (cyclic commit graph) this could pose a problem since the algorithm could end up in a loop. It's non trivial to create such repository but it should probably be fixed anyway to prevent intentional DoS attacks.

It is no longer possible to prevent it with simple map but maybe it's possible to impose some other limit (time, depth). Previous code from Gitea 1.8.0 used to do the processing in parallel and give up after one minute. Not sure what would be rendered on output then.

[codecov-io]

Codecov Report

Merging #6807 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #6807      +/-   ##
==========================================
- Coverage   41.21%   41.21%   -0.01%     
==========================================
  Files         421      421              
  Lines       58064    58059       -5     
==========================================
- Hits        23931    23927       -4     
+ Misses      30966    30965       -1     
  Partials     3167     3167

Impacted Files	Coverage Δ
modules/git/commit_info.go	69.37% <100%> (+0.28%)	⬆️
models/gpg_key.go	55.83% <0%> (-0.84%)	⬇️
models/repo_list.go	67.89% <0%> (+1.05%)	⬆️

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[adelowo]

We have to backport this to 1.8 ?

[filipnavara]

1.8 is not affected