Allow using the rosdistro source version as a track's devel_branch

[nuclearsandwich]

In tracks.yaml, a devel_branch field can be used to automatically scan for and select the latest tag and automatically select it for release. In nearly all cases the devel_branch value should be the same as the repositories[].source.version entry for the corresponding repository in the rosdistro distribution.yaml. These two values becoming de-synchronized is a common trip-up for package maintainers.

It would be nice to add a special value for the devel_branch field similar to the :{auto} and :{version} special values already used for some fields like :{rosdistro_source_version} and have bloom use the value from the rosdistro source so that updates to the branch only need to occur in one place when using this special value.

This is something that we've been talking about for a while but I don't believe it has ever been recorded. If the feature request already exists and my search didn't find it we can close this as a duplicate.