git-gui: Correct file_states when unstaging partly staged entry
When unstaging a partly staged file or submodule, the file_states list was not updated properly (unless unstaged linewise). Its index_info part did not contain the former head_info as it should have but kept its old value.
This seems not to have had any bad effects but diminishes the value of the file_states list for future enhancements.
Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de> Signed-off-by: Shawn O. Pearce <email@example.com>