Avoid a segmentation fault with renaming merges

Under very particular circumstances, merge-recursive's `add_cacheinfo()`
function gets a `NULL` returned from `refresh_cache_entry()` without
expecting it, and subsequently passes it to `add_cache_entry()` which
consequently crashes.

Let's not crash.

This fixes #952

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

merge-recursive.c

@@ -235,7 +235,7 @@ static int add_cacheinfo(struct merge_options *o,
 		struct cache_entry *nce;
 
 		nce = refresh_cache_entry(ce, CE_MATCH_REFRESH | CE_MATCH_IGNORE_MISSING);
-		if (nce != ce)
+		if (nce && nce != ce)
 			ret = add_cache_entry(nce, options);
 	}
 	return ret;

t/t3501-revert-cherry-pick.sh

@@ -141,7 +141,7 @@ test_expect_success 'cherry-pick "-" works with arguments' '
 	test_cmp expect actual
 '
 
-test_expect_failure 'cherry-pick works with dirty renamed file' '
+test_expect_success 'cherry-pick works with dirty renamed file' '
 	test_commit to-rename &&
 	git checkout -b unrelated &&
 	test_commit unrelated &&