…#51076)

Issue: https://github.com/sourcegraph/sourcegraph/issues/50506

The issue manifests for a structural search for any revisions specified
in the query that have both of the following:
1) the revision is a commit and is the latest commit of an indexed
branch in Zoekt
2) the revision is not the branch name indexed in Zoekt (i.e. it is not
a revision specified in `experimentalFeatures.search.index.branches`)

The following fix works for all cases where the indexed branch to query
is `HEAD`. Therefore this will resolve the code insights use case since
code insights repo filter does not accept revisions.

The fix in this PR is intended to be something we can ship quickly and
therefore I hoped to avoid modifying existing logic as much as possible.
For this reason we keep the `SearchJob` arguments and logic unchanged
but just modify the repo revisions passed in.

Note that there is still a remaining issue where if we specify two
revisions for our query and both of those revisions match both criteria
described above, then we do not have enough information to determine
which branch to use since we'll have two branches and neither of them
have a map to their latest commit. To resolve this I believe we'd have
to modify
[`PartitionRepos()`](https://sourcegraph.com/github.com/sourcegraph/sourcegraph/-/blob/internal/search/zoekt/indexed_search.go?L177:6)
and/or
[`IndexedRepoRevs`](https://sourcegraph.com/github.com/sourcegraph/sourcegraph/-/blob/internal/search/zoekt/indexed_search.go?L38)
which would be a bit more involved so I am planning to spin off to a
separate ticket.

## Test plan

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https://docs.sourcegraph.com/dev/background-information/testing_principles
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New method has test cases added

(cherry picked from commit 633c37b)