git-gui: Relabel the Add All action.

One user that I spoke with recently was confused why the 'Add All'
button did not add all of his 'Changed But Not Updated' files.
The particular files in question were new, and thus not known to
Git.  Since the 'Add All' routine only updates files which are
already tracked, they were not added automatically.

I suspect that calling this action 'Add Existing' would be less
confusing, so I'm renaming it.

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

git-gui.sh

@@ -5124,7 +5124,7 @@ if {[is_enabled multicommit] || [is_enabled singlecommit]} {
 	lappend disable_on_lock \
 		[list .mbar.commit entryconf [.mbar.commit index last] -state]
 
-	.mbar.commit add command -label {Add All To Commit} \
+	.mbar.commit add command -label {Add Existing To Commit} \
 		-command do_add_all \
 		-accelerator $M1T-I \
 		-font font_ui
@@ -5423,7 +5423,7 @@ pack .vpane.lower.commarea.buttons.rescan -side top -fill x
 lappend disable_on_lock \
 	{.vpane.lower.commarea.buttons.rescan conf -state}
 
-button .vpane.lower.commarea.buttons.incall -text {Add All} \
+button .vpane.lower.commarea.buttons.incall -text {Add Existing} \
 	-command do_add_all \
 	-font font_ui
 pack .vpane.lower.commarea.buttons.incall -side top -fill x