links to attachments in "attachment created" comments, when exporting/importing issues (BB-10074)

When you have a "log" entry in an issue which includes "attached" and a comment, such as https://bitbucket.org/zzzeek/alembic/issue/107#comment-4100590, the actual link to the attachment in that comment is not necessarily correct. In the case of this specific comment, it appears that the attachment is actually stored under a path that matches a previous name for this issue tracker, "alembic_git". The reason for this appears to be that the hard-coded path of an attachment comes out in an issue export as the "changed_from" field of a "log" entry (which also appears to be undocumented save for a subtle implication in the documentation's example import, and also it makes no sense why "changed_from" would be used for this). It's also impossible (or at least, tedious) to predict what this link would be when making an import from an entirely different issue system like Trac, and if you can predict it, it still appears that the name of the target repo has to be hardwired in the link making the issue export file non-portable as far as links to attachments in comments are concerned.

The way this should work is, the Bitbucket issue itself knows the actual URL of the attachment, it has no problem listing it at the top of the the issue, so it shouldn't need be expressed in literal form in the import or export at all, and the link within the comment should just work just as well as it does on the top of the issue display.

Sorry I know this is a bit picky but I'm considering giving up control of my Trac issues for the first time in 8 years ...