work around bug in git 1.8.2.1 in tests

Making empty commits in tests blocked by opening the default text
editor, waiting for a message which we specified to be empty.

http://thread.gmane.org/gmane.comp.version-control.git/225529

test/--version.bats

@@ -9,7 +9,7 @@ setup() {
 }
 
 git_commit() {
-  git commit --quiet --allow-empty -m "" --allow-empty-message
+  git commit --quiet --allow-empty -m "empty"
 }
 
 @test "default version" {