Trying to stage a commit in an empty repository (i.e. no prior commits)
results in a git fatal error. This is because in an empty repo, HEAD
doesn't point to anything causing `git ls-tree -l HEAD ...` to fail.

To fix this, send `treeish` as `'HEAD'` to `getObjectDetails()` only if
there is at least one commit in the repo. Else, send an empty string
causing `getObjectDetails()` to use `lsFiles` instead of `lsTree`.

Fixes #82026