fix bug pull request files will be broken if head repo was transfered to another user or orgnization

[lunny]

Fix #8563, but this PR will not fix the broken pull requests because the bug because we cannot add a migration on this release branch. There are two workarounds to fix that.

• Transfer the head repository to someplace and transfer it back.
• Update the database to change the head_user_name to the correct one on table pull_request.

This PR is a manually back port from #8564 .

[zeripath]

Would it not be possible to replace every call to headusername with the equivalent looking up from the headrepo id?

[lunny]

@zeripath Did you mean we will not drop that column, but we could do not use that column?

[lafriks]

@lunny that could probably be better

[lafriks]

we can leave that column for 1.9 and drop only in 1.10 but fix not to use it already in 1.9

[lunny]

@lafriks drop column cannot be in v1.10 I think, only in v1.11 since release v1.10 branched.

[lafriks]

It can, as long as no PR is merged that has migration that won't be backported and migrations does not go out of sync for 1.10 and master branches

[lunny]

OK. So please review #8564 at first.