Always return a list from GetCommitsFromIDs

[zeripath]

GetCommitsFromIDs is only used in one place: LoadPushCommits where
it expects that c.Commits is not nil.

This potentially nil set causes a NPE in in #12953

Fix #12953

Signed-off-by: Andrew Thornton art27@cantab.net

[codecov-commenter]

Codecov Report

Merging #12981 into master will increase coverage by 0.12%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #12981      +/-   ##
==========================================
+ Coverage   42.47%   42.59%   +0.12%     
==========================================
  Files         671      671              
  Lines       73627    73625       -2     
==========================================
+ Hits        31272    31362      +90     
+ Misses      37291    37188     -103     
- Partials     5064     5075      +11     

Impacted Files	Coverage Δ
modules/git/repo_commit.go	50.91% <ø> (+0.37%)	⬆️
models/repo_mirror.go	2.38% <0.00%> (-11.91%)	⬇️
modules/charset/charset.go	68.53% <0.00%> (-4.50%)	⬇️
modules/cron/tasks_basic.go	87.35% <0.00%> (-3.45%)	⬇️
modules/log/event.go	56.60% <0.00%> (-2.84%)	⬇️
services/mirror/mirror.go	18.51% <0.00%> (-1.35%)	⬇️
services/pull/pull.go	40.78% <0.00%> (+0.49%)	⬆️
models/error.go	35.19% <0.00%> (+0.84%)	⬆️
models/notification.go	66.66% <0.00%> (+0.90%)	⬆️
modules/process/manager.go	75.00% <0.00%> (+2.50%)	⬆️
... and 9 more

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