Rename is-develop to track-releases-branches

[qetza]

As proposed in the PR #1004 i think it's a good idea to rename is-develop to track-release-branches because you could want the tracking mechanism on other branches and the name would confuse users.

The current changes was done in the the PR but mixed with other changes and the PR is still not merged. Maybe we could extract this to a dedicated PR so that in can be included in 4.0.0 release?

[stale]

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