Merge branch 'jk/has-uncommitted-changes-fix'

"git pull --rebase" on a corrupt HEAD caused a segfault. In general we substitute an empty tree object when running the in-core equivalent of the diff-index command, and the codepath has been corrected to do so as well to fix this issue. * jk/has-uncommitted-changes-fix: has_uncommitted_changes(): fall back to empty tree