index: overwrite the path when inserting conflicts

When we insert a conflict in a case-insensitive index, accept the
new entry's path as the correct case instead of leaving the path we
already had.

This puts `git_index_conflict_add()` on the same level as
`git_index_add()` in this respect.

CHANGELOG.md

@@ -41,10 +41,11 @@ v0.23 + 1
   with which to implement the transactional/atomic semantics for the
   configuration backend.
 
-* `git_index_add` will now use the case as provided by the caller on
-  case insensitive systems.  Previous versions would keep the case as
-  it existed in the index.  This does not affect the higher-level
-  `git_index_add_bypath` or `git_index_add_frombuffer` functions.
+* `git_index_add` and `git_index_conflict_add()` will now use the case
+  as provided by the caller on case insensitive systems.  Previous
+  versions would keep the case as it existed in the index.  This does
+  not affect the higher-level `git_index_add_bypath` or
+  `git_index_add_frombuffer` functions.
 
 * The `notify_payload` field of `git_diff_options` was renamed to `payload`
   to reflect that it's also the payload for the new progress callback.

src/index.c

@@ -1718,7 +1718,7 @@ int git_index_conflict_add(git_index *index,
 		/* Make sure stage is correct */
 		GIT_IDXENTRY_STAGE_SET(entries[i], i + 1);
 
-		if ((ret = index_insert(index, &entries[i], 0, true, true)) < 0)
+		if ((ret = index_insert(index, &entries[i], 1, true, true)) < 0)
 			goto on_error;
 
 		entries[i] = NULL; /* don't free if later entry fails */