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: NJens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>

git-gui.sh

@@ -1613,6 +1613,9 @@ proc merge_state {path new_state {head_info {}} {index_info {}}} {
 	} elseif {$s0 ne {_} && [string index $state 0] eq {_}
 		&& $head_info eq {}} {
 		set head_info $index_info
+	} elseif {$s0 eq {_} && [string index $state 0] ne {_}} {
+		set index_info $head_info
+		set head_info {}
 	}
 
 	set file_states($path) [list $s0$s1 $icon \