This change is going to cause some unfortunate problems for people with a ucsc_build_sites.txt that has been modified. Ideally we could simply "forget" the file and leave it unmodified in downstream clones. Unfortunately, that does not seem to be possible.
As a result of this changeset, the following will occur:
In clones with no modifications: ucsc_build_sites.txt will disappear. run.sh will recreate it at next run, however, those not using run.sh will need to recreate ucsc_build_sites.txt from ucsc_build_sites.txt.sample manually.
In clones with modifications: The downstream ucsc_build_sites.txt and upstream ucsc_build_sites.txt.sample will be merged into ucsc_build_sites.txt.sample and ucsc_build_sites.txt will disappear. In this case, the admin should make a backup of ucsc_build_sites.txt before updating and copy it back in place after updating (or use the merged ucsc_build_sites.txt.sample to recreate ucsc_build_sites.txt). The (uncommitted) changes to ucsc_build_sites.txt.sample can be reverted once ucsc_build_sites.txt is restored.
I tried the alternatives of hg forget or various hg rm incantations and none of them produced the desired result.