wp-cli/wp-cli — pull request #1320 by danielbachhuber (2014-08-21 18:40:41+00:00)
Title: Core updates should be stored in the cache directory

I ran wp core update for 3.9.1 => 3.9.2 on ~75 sites on my server.
The upgrade files were not stored in .wp-cli/cache/core, and it appeared that a fresh download of the changed files were downloaded for each of the 75 sites.

message in CLI was:
Downloading update from https://downloads.wordpress.org/release/wordpress-3.9.2-partial-1.zip
Unpacking the update...
Success: WordPress updated successfully

Re-tested after clearing .wp-cli/cache/core and no new files were placed there.