When I last updated the Cygwin git, it stopped working on my Samba drive. Normally running git on a network drive is not recommended, but I do it anyway. After a git upgrade, I started seeing:
error: invalid object error: Error building trees
when I did a commit. After a little searching, I discovered this Stack Overflow post which suggested one answer:
git config --add "core.createobject" rename
This appears to have solved the problem for me.