Sync with Git 2.15.2

* maint-2.15: Git 2.15.2 Git 2.14.4 Git 2.13.7 verify_path: disallow symlinks in .gitmodules update-index: stat updated files earlier verify_dotfile: mention case-insensitivity in comment verify_path: drop clever fallthrough skip_prefix: add case-insensitive variant is_{hfs,ntfs}_dotgitmodules: add tests is_ntfs_dotgit: match other .git files is_hfs_dotgit: match other .git files is_ntfs_dotgit: use a size_t for traversing string submodule-config: verify submodule names as paths