omicron-git-version requires rebuilding the most expensive-to-rebuild crate in the tree every time the git HEAD changes

omicron-git-version, introduced in #10578, is depended on by nexus-db-model and omdb. This uses vergen-gitcl, which emits (among other things) cargo:rerun-if-changed=.git/HEAD in a build script.

This causes all downstream crates of nexus-db-model to be recompiled any time any git operation is done which moves the HEAD, e.g. checkout or commit; this includes rebuilding nexus-db-schema, which is very expensive to recompile.

In particular I have noticed this to be particularly egregious while working on releng, amending a commit, and then retesting releng -- the system now has to rebuild half of Nexus which would have otherwise already been fresh.

I don't really have any suggestions for a better option yet, but I wanted to track this somewhere.

cc @hawkw @smklein

Start by inspecting the build script for omicron-git-version and how vergen-gitcl emits the .git/HEAD rerun directive, then trace its dependency paths through nexus-db-model, omdb, and nexus-db-schema. Measure rebuilds after a checkout or commit; done means HEAD changes no longer unnecessarily recompile the expensive downstream crates.