git-gui: Call changes "Staged" and "Unstaged" in file list titles.

All menu entries talk about "staging" and "unstaging" changes, but the
titles of the file lists use different wording, which may confuse
newcomers.

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

git-gui.sh

@@ -5496,7 +5496,7 @@ pack .vpane -anchor n -side top -fill both -expand 1
 # -- Index File List
 #
 frame .vpane.files.index -height 100 -width 200
-label .vpane.files.index.title -text {Changes To Be Committed} \
+label .vpane.files.index.title -text {Staged Changes (Will Be Committed)} \
 	-background green \
 	-font font_ui
 text $ui_index -background white -borderwidth 0 \
@@ -5518,7 +5518,7 @@ pack $ui_index -side left -fill both -expand 1
 # -- Working Directory File List
 #
 frame .vpane.files.workdir -height 100 -width 200
-label .vpane.files.workdir.title -text {Changed But Not Updated} \
+label .vpane.files.workdir.title -text {Unstaged Changes (Will Not Be Committed)} \
 	-background red \
 	-font font_ui
 text $ui_workdir -background white -borderwidth 0 \