Make it possible to cause the reimport of a single Git-based OSV record

[andrewpollock]

Problem statement:

Today, it is not possible to cause the reimport (and re-enumeration) of a single record in a Git-based OSV record source.
The best option (as opposed to setting last_synced_hash to null for the given source record in SourceRepository in Datastore, and causing a full reimport) is to determine the commit before the commit that added/modified the record of interest in the Git repository, and reset last_synced_hash to that commit hash. Depending on how far back in time this is, the amount of unnecessary reprocessing is significant (although this will be less relevant once #2017 is implemented, and if the interval there is sufficiently short it will reduce the need for this to be addressed).