IndexPack: Defer the "Resolving deltas" progress meter

IndexPack: Defer the "Resolving deltas" progress meter

If delta resolution completes in < 1000 milliseconds, don't bother
showing the progress meter. This is actually very common for a Gerrit
Code Review server, where the client is probably sending 1 commit and
only a few trees/blobs modified... and the base objects are hot in the
process buffer cache.

The 1000 millisecond delay is just a guess at a reasonable time to wait.

Change-Id: I440baa64ab0dfa21be61deae8dcd3ca061bed8ce
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>