Fix markdown meta parsing

[jolheiser]

Fixes #12815

This code should now exclude the separator lines themselves, so as long as there are two or more hyphens it will work as a separator and also YAML won't choke.

Included a minimal test case.

[codecov-commenter]

Codecov Report

Merging #12817 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #12817      +/-   ##
==========================================
+ Coverage   43.13%   43.16%   +0.02%     
==========================================
  Files         654      654              
  Lines       72205    72205              
==========================================
+ Hits        31146    31165      +19     
+ Misses      36013    35990      -23     
- Partials     5046     5050       +4     

Impacted Files	Coverage Δ
modules/markup/markdown/meta.go	90.90% <100.00%> (+90.90%)	⬆️
modules/process/manager.go	72.50% <0.00%> (-2.50%)	⬇️
modules/queue/unique_queue_disk_channel.go	53.84% <0.00%> (-1.54%)	⬇️
models/repo_list.go	78.57% <0.00%> (-0.90%)	⬇️
services/pull/pull.go	41.57% <0.00%> (-0.47%)	⬇️
modules/log/event.go	57.54% <0.00%> (+0.94%)	⬆️
models/unit.go	49.31% <0.00%> (+2.73%)	⬆️
modules/util/timer.go	85.71% <0.00%> (+42.85%)	⬆️

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[lunny]

It's better to use Scanner.

scanner := bufio.NewScanner(strings.NewReader(contents))
var idx = 0 
for scanner.Scan() {
    line := scanner.Text()
    ...
    idx++
}

[jolheiser]

What's the advantage in this case?
We still need to scan the whole file so we can return body, and we've already checked file size prior to calling this func.

[lunny]

Less CPU and memory usage. But since it should be a small file, it's not necessary.