Fix sample logic when adding content files in server

Fix sample logic when adding content files in server

The partial rebuilds works by calaulating a baseline from a change set.

For new content, this doesn't work, so to avoid rebuilding everything, we first
try to collect a sample of surrounding identities (e.g. content files in the same section).

This commit fixes a flaw in that logic that in some (many...) cases would return a too small sample set.

Fixes #12054