- The content history table contains the content history of issues and
comments. For issues they are saved with an comment id of zero.
- If you want to check if the issue has an content history, it should
take into account that SQL has `comment_id = 0`, as it otherwise could
return incorrect results when for example the issue already has an
comment that has an content history.
- Fix the code of `HasIssueContentHistory` to take this into account, it
relied on XORM to generate the SQL from the non-default values of the
struct, this wouldn't generate the `comment_id = 0` SQL as `0` is the
default value of an integer.
- Remove an unncessary log (it's not the responsibility of `models`
code to do logging).
- Adds unit test.
- Resolves#2513
331fa44956