mergetools/vimdiff: trust Vim's exit code

Allow vimdiff users to signal that they do not want to use the result of a merge by exiting with ":cquit", which tells Vim to exit with an error code.

This is better than the current behavior because it allows users to directly flag that the merge is bad, using a standard Vim feature, rather than relying on a timestamp heuristic that is unforgiving to users that save in-progress merge files.

The original behavior can be restored by configuring mergetool.vimdiff.trustExitCode to false.

---
I've included anyone that has ever touched the vimdiff feature on the Cc:
list since I'm assuming that you use vimdiff.

This change is a slight change in default behavior when using mergetool with vimdiff, but I think it's a better default overall.

 mergetools/vimdiff | 4 ++++
 1 file changed, 4 insertions(+)

index a841ffdb4..10d86f3e1 100644
@@ -42,3 +42,7 @@ translate_merge_tool_path() {
 		;;
 	esac
 }
+
+exit_code_trustable () {
+	true
+}

2.11.0.rc3.6.g2e567fd