Merge branch 'jc/fix-diff-files-unmerged' into maint

* jc/fix-diff-files-unmerged:
  diff-files: show unmerged entries correctly
  diff: remove often unused parameters from diff_unmerge()
  diff.c: return filepair from diff_unmerge()
  test: use $_z40 from test-lib

t/test-lib.sh

@@ -89,6 +89,9 @@ esac
 _x05='[0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f]'
 _x40="$_x05$_x05$_x05$_x05$_x05$_x05$_x05$_x05"
 
+# Zero SHA-1
+_z40=0000000000000000000000000000000000000000
+
 # Each test should start with something like this, after copyright notices:
 #
 # test_description='Description of this test...