Remove flush_odb_transaction() from bulk-checkin subsystem

The bulk-checkin subsystem provides an interface to manage object database transactions. A transaction is started with begin_odb_transaction() and ended with end_odb_transaction(). Interestingly, there is also flush_odb_transaction(), which migrates objects written during the transaction to the primary object database without ending the transaction.

This makes for a rather awkward interface as generally ending/committing a transaction should be the point where things actually get committed. We should drop flush_odb_transaction() and enforce that a transaction only commits when ended.

There is currently only a single caller of flush_odb_transaction() in git-update-index(1). It is used when the command is specified with the --verbose option so that objects are immediately made visible when the output reports they are written. This is really only useful when also paired with --stdin. We will need to adapt this single call site to either not use transactions or commit and start a new one.