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Changes between Initial Version and Version 1 of Ticket #54800, comment 3

initial	v1
2	2	2. In order to cache dependencies across PRs, we have to enable builds on the master branch. To avoid timeouts, we can only build ports changed in the last commit pushed. (When many unrelated commits are pushed at once, only one Travis build will be triggered.)
3	3	3. We should not cache distributable packages and purge outdated cached packages to avoid making the cache too big. Extracting the cache (and maybe generating it) costs precious build time.
	4
	5	> After the first pull request build is run, it creates a new pull request cache.
	6	4. We have to remove cache of ports to be built.