git-gui: Support file state MD (modified/deleted).

Apparently I missed the file state MD, which is a file modified and
updated in the index but then removed from the working directory.  This
should be treated just like AD, an added file which has been deleted from
the working directory.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

git-gui

@@ -1093,6 +1093,7 @@ proc commit_committree {fd_wt curHEAD msg} {
 		AM -
 		AD -
 		MM -
+		MD -
 		DM {
 			set file_states($path) [list \
 				_[string index $m 1] \
@@ -2211,6 +2212,7 @@ proc include_helper {txt paths} {
 		AM -
 		AD -
 		MM -
+		MD -
 		U? -
 		_M -
 		_D -
@@ -2255,6 +2257,7 @@ proc do_include_all {} {
 		AM -
 		AD -
 		MM -
+		MD -
 		_M -
 		_D {lappend paths $path}
 		}