path: using .relative() should not return a trailing slash

Resolving a path against root with `path.relative()` should not
include a trailing slash.

Fixes: #28549

PR-URL: #28556
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>

lib/path.js

@@ -1090,11 +1090,16 @@ const posix = {
           // For example: from='/'; to='/foo'
           return to.slice(toStart + i);
         }
-      } else if (fromLen > length &&
-                 from.charCodeAt(fromStart + i) === CHAR_FORWARD_SLASH) {
-        // We get here if `to` is the exact base path for `from`.
-        // For example: from='/foo/bar/baz'; to='/foo/bar'
-        lastCommonSep = i;
+      } else if (fromLen > length) {
+        if (from.charCodeAt(fromStart + i) === CHAR_FORWARD_SLASH) {
+          // We get here if `to` is the exact base path for `from`.
+          // For example: from='/foo/bar/baz'; to='/foo/bar'
+          lastCommonSep = i;
+        } else if (i === 0) {
+          // We get here if `to` is the root.
+          // For example: from='/foo/bar'; to='/'
+          lastCommonSep = 0;
+        }
       }
     }
 

test/parallel/test-path-relative.js

@@ -47,7 +47,8 @@ const relativeTests = [
      ['/foo/bar/baz-quux', '/foo/bar/baz', '../baz'],
      ['/foo/bar/baz', '/foo/bar/baz-quux', '../baz-quux'],
      ['/baz-quux', '/baz', '../baz'],
-     ['/baz', '/baz-quux', '../baz-quux']
+     ['/baz', '/baz-quux', '../baz-quux'],
+     ['/page1/page2/foo', '/', '../../..']
     ]
   ]
 ];