Do not apply exclusions to unrelated paths ("project", "proxy.db" and
"etc").
Simplify global exclusion file generation.
Add support for absolute and partition-relative exclusion paths
(previously, only partition-relative were supported).
Pipe exclusion file to rsync, to avoid an intermediate disk write.
Remove some directory prefix duplication by using PWD-relative paths
(except in one place, for runner-import.sh.jinja2 comparison).