Sync with Git 2.13.7

* maint-2.13:
  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

submodule-config.h

@@ -48,4 +48,11 @@ extern int gitmodule_sha1_from_commit(const unsigned char *commit_sha1,
 				      struct strbuf *rev);
 extern void submodule_free(void);
 
+/*
+ * Returns 0 if the name is syntactically acceptable as a submodule "name"
+ * (e.g., that may be found in the subsection of a .gitmodules file) and -1
+ * otherwise.
+ */
+int check_submodule_name(const char *name);
+
 #endif /* SUBMODULE_CONFIG_H */