Fix sync tool bugs (#1028)

* fix: filter out empty lines
update: test with newlines

* add: git repo check and warnings
update: println -> eprintln
feature: allow commit total to be limited
add: warning for leftover commits to sync
fix: create commit hash file if none exists
fix: get working dir in a github-actions-friendly way
update: add more context
fix: clippy lints

* update: simplify file writing
update: remove redundant lifetimes

Co-authored-by: Russell Cohen <rcoh@amazon.com>