cleanup code getIssueFullPattern in modules/markup

[Dexus]

fix #16415

[zeripath]

I think this doesn't need to go in to 1.15 - the "race" is somewhat theoretical as we can assert that Init() gets called and completes before anything else would call the get* fn. There's likely to be another Once we can use to for setting these regexps too but I'll have to have a deeper look,

[codecov-commenter]

Codecov Report

Merging #16419 (8ab468b) into main (0393a57) will increase coverage by 1.18%.
The diff coverage is 49.05%.

@@            Coverage Diff             @@
##             main   #16419      +/-   ##
==========================================
+ Coverage   44.24%   45.42%   +1.18%     
==========================================
  Files         695      717      +22     
  Lines       82341    84108    +1767     
==========================================
+ Hits        36429    38206    +1777     
+ Misses      40014    39749     -265     
- Partials     5898     6153     +255     

Impacted Files	Coverage Δ
cmd/convert.go	0.00% <0.00%> (ø)
cmd/dump.go	0.91% <0.00%> (ø)
cmd/dump_repo.go	0.00% <0.00%> (ø)
cmd/generate.go	0.00% <0.00%> (ø)
cmd/hook.go	0.00% <0.00%> (ø)
cmd/migrate.go	0.00% <0.00%> (ø)
cmd/migrate_storage.go	0.00% <0.00%> (ø)
cmd/serv.go	2.36% <0.00%> (-0.22%)	⬇️
cmd/web.go	0.00% <0.00%> (ø)
cmd/web_letsencrypt.go	0.00% <0.00%> (ø)
... and 307 more

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