merge-recursive.c: fix case-changing merge bug

On a case-insensitive filesystem, when merging, a file would be
wrongly deleted from the working tree if an incoming commit had
renamed it changing only its case.  When merging a rename, the file
with the old name would be deleted -- but since the filesystem
considers the old name to be the same as the new name, the new
file would in fact be deleted.

We avoid this by not deleting files that have a case-clone in the
index at stage 0.

Signed-off-by: David Turner <dturner@twitter.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

merge-recursive.c

@@ -589,6 +589,12 @@ static int remove_file(struct merge_options *o, int clean,
 			return -1;
 	}
 	if (update_working_directory) {
+		if (ignore_case) {
+			struct cache_entry *ce;
+			ce = cache_file_exists(path, strlen(path), ignore_case);
+			if (ce && ce_stage(ce) == 0)
+				return 0;
+		}
 		if (remove_path(path))
 			return -1;
 	}

t/t6039-merge-ignorecase.sh

@@ -0,0 +1,53 @@
+#!/bin/sh
+
+test_description='git-merge with case-changing rename on case-insensitive file system'
+
+. ./test-lib.sh
+
+if ! test_have_prereq CASE_INSENSITIVE_FS
+then
+	skip_all='skipping case insensitive tests - case sensitive file system'
+	test_done
+fi
+
+test_expect_success 'merge with case-changing rename' '
+	test $(git config core.ignorecase) = true &&
+	>TestCase &&
+	git add TestCase &&
+	git commit -m "add TestCase" &&
+	git tag baseline
+	git checkout -b with-camel &&
+	>foo &&
+	git add foo &&
+	git commit -m "intervening commit" &&
+	git checkout master &&
+	git rm TestCase &&
+	>testcase &&
+	git add testcase &&
+	git commit -m "rename to testcase" &&
+	git checkout with-camel &&
+	git merge master -m "merge" &&
+	test_path_is_file testcase
+'
+
+test_expect_success 'merge with case-changing rename on both sides' '
+	git checkout master &&
+	git reset --hard baseline &&
+	git branch -D with-camel &&
+	git checkout -b with-camel &&
+	git mv --force TestCase testcase &&
+	git commit -m "recase on branch" &&
+	>foo &&
+	git add foo &&
+	git commit -m "intervening commit" &&
+	git checkout master &&
+	git rm TestCase &&
+	>testcase &&
+	git add testcase &&
+	git commit -m "rename to testcase" &&
+	git checkout with-camel &&
+	git merge master -m "merge" &&
+	test_path_is_file testcase
+'
+
+test_done