DfsPackFile: Record index loads only in one place

DfsPackFile: Record index loads only in one place

Each index can be set in the reader from two locations: the dfs cache
callback or the code afterwards. The pack is emitting the load event
in both cases, when the reference is set. This is brittle (right now
it is missing events for BITMAP_INDEX and COMMIT_GRAPH).

Emit the index loaded event only once, after going through the cache
code. The fact that the reference was set in the callback or the main
code is irrelevant. Also, the reader is per-thread, so there shouldn't
be any concurrency involved triggering double counts.

Change-Id: I7f3d078a53741ecc1e81b96353ed8faa8fef3a49