Defer object flushing when merging

Mergers in the most recent JGit know how to read previously-inserted
objects back, so a series of merges does not require aggressively
flushing the objects. Instead, flush objects in MergeOp just before
updating the relevant refs.

Change-Id: Ia85c5114e86739b4a480bea65f3413fe47cf572d
5 files changed