Fix bug on migration 111

[lunny]

The sess.Begin have to be invoked if sess.Commit will be invoked.

From https://drone.gitea.io/go-gitea/gitea/29887/2/13

[zeripath]

Interesting pg is declaring a failed transaction on the 1.6.4 migration.

[codecov-commenter]

Codecov Report

Merging #12736 into master will increase coverage by 0.00%.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master   #12736   +/-   ##
=======================================
  Coverage   43.40%   43.41%           
=======================================
  Files         648      648           
  Lines       71569    71581   +12     
=======================================
+ Hits        31067    31076    +9     
- Misses      35472    35479    +7     
+ Partials     5030     5026    -4     

Impacted Files	Coverage Δ
models/migrations/v111.go	0.00% <0.00%> (ø)
modules/log/event.go	57.54% <0.00%> (-0.95%)	⬇️
services/pull/pull.go	42.72% <0.00%> (+1.15%)	⬆️
services/pull/temp_repo.go	29.78% <0.00%> (+3.19%)	⬆️
services/pull/update.go	57.69% <0.00%> (+5.76%)	⬆️

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[lunny]

A blank transaction will cause failure on postgres. So we have to check update times before commit.