Merge branch 'bw/realpath-wo-chdir' into next

Merge branch 'bw/realpath-wo-chdir' into next

The implementation of "real_path()" was to go there with chdir(2)
and call getcwd(3), but this obviously wouldn't be usable in a
threaded environment.  Rewrite it to manually resolve relative
paths including symbolic links in path components.

* bw/realpath-wo-chdir:
  real_path: canonicalize directory separators in root parts
  real_path: have callers use real_pathdup and strbuf_realpath
  real_path: create real_pathdup
  real_path: convert real_path_internal to strbuf_realpath
  real_path: resolve symlinks by hand