Add source_modified() that uses git ls-tree -r HEAD to hash the
contents of all tracked files in a source dir, falling back to
modified_dir_ignore_git for non-git sources. Wire into cook_build.rs
in place of the recursive mtime walk.
Eliminates spurious rebuilds from:
- .swp files, editor backups
- build artifacts in target/ or other untracked dirs
- filesystem timestamp drift after touch operations
The git-tree approach is content-addressed: identical content always
yields the same fingerprint, regardless of mtime. Combined with T1.1
(sysroot content-hash + pkgar mtime preservation), a no-op rebuild
should take seconds rather than hours.
Plan: local/docs/BUILD-SYSTEM-ROBUSTNESS-PLAN.md
Red Bear CI
088d3c745d