git-gui: Ensure consistent usage of mergetool.keepBackup

In several places merge.keepBackup is used i.s.o.
mergetool.keepBackup. This patch makes it all
consistent.
Signed-off-by: NFerry Huberts <ferry.huberts@pelagic.nl>
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>

git-gui.sh

@@ -699,7 +699,7 @@ proc apply_config {} {
 
 set default_config(branch.autosetupmerge) true
 set default_config(merge.tool) {}
-set default_config(merge.keepbackup) true
+set default_config(mergetool.keepbackup) true
 set default_config(merge.diffstat) true
 set default_config(merge.summary) false
 set default_config(merge.verbosity) 2

lib/mergetool.tcl

@@ -382,7 +382,7 @@ proc merge_tool_finish {fd} {
 		delete_temp_files $mtool_tmpfiles
 		ui_status [mc "Merge tool failed."]
 	} else {
-		if {[is_config_true merge.keepbackup]} {
+		if {[is_config_true mergetool.keepbackup]} {
 			file rename -force -- $backup "$mtool_target.orig"
 		}