fix bug when get tag id

[lunny]

This will fix go-gitea/gitea#5585

It seems

git show-ref --tags <tag>

will not give the right commit id of this tag, but

git rev-list -n 1 $TAG

will return the right commit id.

[codecov-io]

Codecov Report

Merging #147 into master will decrease coverage by 1.37%.
The diff coverage is 22.22%.

@@            Coverage Diff            @@
##           master    #147      +/-   ##
=========================================
- Coverage   37.28%   35.9%   -1.38%     
=========================================
  Files          28      28              
  Lines        1832    1838       +6     
=========================================
- Hits          683     660      -23     
- Misses       1055    1089      +34     
+ Partials       94      89       -5

Impacted Files	Coverage Δ
repo_commit.go	24.72% <0%> (-0.56%)	⬇️
repo_tag.go	36.36% <100%> (+4.04%)	⬆️
tag.go	10.86% <0%> (-58.7%)	⬇️

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[lunny]

@thibaultmeyer could you confirm this will fix your issue?