repository: Ignore missing refs in CreateBundleFromRefList

Fixes #4000 (closed)

This RPC is used to create backup bundles. The list of refs passed in is collected by a call to ListRefs. If a ref is deleted between these two calls then the git bundle create call made by CreateBundleFromRefList would fail with an error.

We are not interested in missing refs for backups because there will be no need to restore a now deleted ref. However, ignoring missing refs will mean that the list of refs saved for incremental backups will be out-of-date compared to the bundle. There should be no problem creating incremental backups because we only need the object ID that the ref used to be pointed at. So the ref names would only be used to detect ref deletions from the previous backup on restore (#3857), any extra refs in the ref list can simply be ignored.