Fix manifest generation to preserve non-e repo revisions when REGENERATE_MANIFEST=false

When REGENERATE_MANIFEST=false, the pipeline should only update e/e-priv repository revisions
and preserve existing revisions for LineageOS/AOSP repos. The previous logic using git diff
with ignore patterns was unreliable and caused unintended updates to non-e repositories.

The fix implements proper XML-based selective manifest generation:
- Backs up current manifest to preserve existing non-e repo revisions
- Generates full manifest from repo sync to get latest e/e-priv revisions
- Uses xmlstarlet to selectively update only e/e-priv projects in the backup manifest
- Results in a manifest with updated e/e-priv repos and unchanged non-e repos