Refactor get tag to remove unnecessary steps

[lunny]

This PR removed some unnecessary codes from getTag and also fix a bug related with tagCache. A not about that:

Lightweight git tags may point to the same commit.

[codecov-io]

Codecov Report

Merging #14058 (023f723) into master (4ef7512) will decrease coverage by 0.03%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master   #14058      +/-   ##
==========================================
- Coverage   42.32%   42.29%   -0.04%     
==========================================
  Files         726      726              
  Lines       77684    77677       -7     
==========================================
- Hits        32883    32852      -31     
- Misses      39403    39435      +32     
+ Partials     5398     5390       -8     

Impacted Files	Coverage Δ
modules/git/repo_tag.go	59.54% <75.00%> (+0.84%)	⬆️
modules/git/error.go	25.42% <0.00%> (-28.82%)	⬇️
modules/git/repo_base_nogogit.go	63.63% <0.00%> (-9.10%)	⬇️
modules/git/repo.go	39.41% <0.00%> (-4.12%)	⬇️
modules/log/event.go	58.96% <0.00%> (-2.84%)	⬇️
modules/git/utils.go	77.77% <0.00%> (-2.78%)	⬇️
modules/git/blame.go	65.71% <0.00%> (-1.43%)	⬇️
services/pull/pull.go	40.14% <0.00%> (-1.24%)	⬇️
modules/indexer/stats/db.go	48.00% <0.00%> (ø)
models/error.go	38.98% <0.00%> (+0.48%)	⬆️
... and 2 more

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