Fixes#2691
This is changing the trimming of the merge message template to allow first empty line to be considered as empty message (also referred as title in frontend). 2 unit tests were added to check for proper behavior and possible edge case (simple template with only 2 lines, not trimming needed). Also a former unit test with 3 empty lines at the beginning of the template was changed.
The behavior with this patch is:
* splitting first `\n` to separate message from body
* trim remaining `\n` from body if `\n` was present or take template as is as message (no trim necessary as no `\n` is present in the template)
As this is an old issue the expected behavior might have changed from when the issue was triaged.
I additionally manually testes on the next.forgejo.org and locally built binary from this branch, I am attaching both screenshot of final result. The test PR can be seen at [https://v13.next.forgejo.org/kajika/merge_template_issue_2691/pulls/1](https://v13.next.forgejo.org/kajika/merge_template_issue_2691/pulls/1) and, in case the next repository is deletate the content of `.forgejo/default_merge_message/MERGE_TEMPLATE.md` is (first line is empty):
```
This is the description of the merge from "${HeadBranch}" to "${BaseBranch}"
```
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/9930
Reviewed-by: oliverpool <oliverpool@noreply.codeberg.org>
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Co-authored-by: Corentin <corentin@ayo.tokyo>
Co-committed-by: Corentin <corentin@ayo.tokyo>
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