Merge branch 'nd/clone-case-smashing-warning' into pu

Running "git clone" against a project that contain two files with
pathnames that differ only in cases on a case insensitive
filesystem would result in one of the files lost because the
underlying filesystem is incapable of holding both at the same
time.  An attempt is made to detect such a case and warn.

Discussion getting petered out.
Doing this portably and extending it to UTF-8 normalization issue
HFS+ has would be costly.

cf. <20180728095659.GA21450@sigill.intra.peff.net>
cf. <xmqq1sbh7phx.fsf@gitster-ct.c.googlers.com>

* nd/clone-case-smashing-warning:
  clone: report duplicate entries on case-insensitive filesystems

Author: gitster

builtin/clone.c

@@ -712,6 +712,33 @@ static void update_head(const struct ref *our, const struct ref *remote,
 	}
 }
 
+static void find_duplicate_icase_entries(struct index_state *istate,
+					 struct string_list *dup)
+{
+	struct string_list list = STRING_LIST_INIT_NODUP;
+	int i;
+
+	for (i = 0; i < istate->cache_nr; i++)
+		string_list_append(&list, istate->cache[i]->name);
+
+	list.cmp = fspathcmp;
+	string_list_sort(&list);
+
+	for (i = 1; i < list.nr; i++) {
+		const char *cur = list.items[i].string;
+		const char *prev = list.items[i - 1].string;
+
+		if (dup->nr &&
+		    !fspathcmp(cur, dup->items[dup->nr - 1].string)) {
+			string_list_append(dup, cur);
+		} else if (!fspathcmp(cur, prev)) {
+			string_list_append(dup, prev);
+			string_list_append(dup, cur);
+		}
+	}
+	string_list_clear(&list, 0);
+}
+
 static int checkout(int submodule_progress)
 {
 	struct object_id oid;
@@ -762,6 +789,20 @@ static int checkout(int submodule_progress)
 	if (write_locked_index(&the_index, &lock_file, COMMIT_LOCK))
 		die(_("unable to write new index file"));
 
+	if (ignore_case) {
+		struct string_list dup = STRING_LIST_INIT_DUP;
+		int i;
+
+		find_duplicate_icase_entries(&the_index, &dup);
+		if (dup.nr) {
+			warning(_("the following paths in this repository only differ in case and will\n"
+				  "cause problems because you have cloned it on an case-insensitive filesytem:\n"));
+			for (i = 0; i < dup.nr; i++)
+				fprintf(stderr, "\t%s\n", dup.items[i].string);
+		}
+		string_list_clear(&dup, 0);
+	}
+
 	err |= run_hook_le(NULL, "post-checkout", sha1_to_hex(null_sha1),
 			   oid_to_hex(&oid), "1", NULL);