git-gui: Correct file_states when unstaging partly staged entry

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 <redacted>
Signed-off-by: Shawn O. Pearce <redacted>