Backport #21352

Due to a bug in presumably an older version of Gitea, multiple of my
repositories still have their HEADs pointing to a `master` branch while
the default branch on the UI is listed as `main`. This adds a `gitea
doctor` command that will fix all of the HEAD references for repos when
they're not synchronized with the default branch in the DB.

This will help with cloning to ensure that git automatically checks out
the right branch, instead of a nonexistent one.

Note: I'm not sure if I actually need to do more other than add a file
here. Will try testing this out on my server soon.

Co-authored-by: zeripath <[email protected]>
Co-authored-by: wxiaoguang <[email protected]>