Fix restore repo bug, clarify the problem of ForeignIndex (#22776) (#22794)

Fix restore repo bug, clarify the problem of ForeignIndex (#22776) (#22794)

Backport #22776

Fix #22581

TLDR: #18446 made a mess with ForeignIndex and triggered a design
flaw/bug of #16356, then a quick patch #21271 helped #18446, then the
the bug was re-triggered by #21721 .

Related:
* #16356
* BasicIssueContext
https://github.com/go-gitea/gitea/pull/16356/files#diff-7938eb670d42a5ead6b08121e16aa4537a4d716c1cf37923c70470020fb9d036R16-R27
* #18446
* If some issues were dumped without ForeignIndex, then they would be
imported as ForeignIndex=0
https://github.com/go-gitea/gitea/pull/18446/files#diff-1624a3e715d8fc70edf2db1630642b7d6517f8c359cc69d58c3958b34ba4ce5eR38-R39
* #21271
* It patched the above bug (somewhat), made the issues without
ForeignIndex could have the same value as LocalIndex
* #21721
    * It re-triggered the zero-ForeignIndex bug.

ps: I am not sure whether the changes in `GetForeignIndex` are ideal (at
least, now it has almost the same behavior as BasicIssueContext in
#16356), it's just a quick fix. Feel free to edit on this PR directly or
replace it.

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>