Avoid indexing twice when pull mirroring

The following discussion from !14413 (merged) should be addressed:

• @nick.thomas started a discussion: (+3 comments)

  Hmmm. I'm actually starting to wonder if we need this code at all.

  Does a pull mirror running cause the git repository's post-receive hook to run? If so, then we'll be doing double work here. Perhaps we can remove this code altogether? We had a similar case for wiki indexing, for instance.

  It would need careful manual testing to verify, since the hooks don't run in the gitlab-ee test suite :(

Findings

• Pull mirroring does trigger post-receive
• Importing does not trigger post-receive
• On the project import form we have a checkbox to enable mirroring at the same time

Possible solutions

• Making the hooks run in the import case
• Distinguishing between the two cases in ProjectImportState