real_path: resolve symlinks by hand

real_path: resolve symlinks by hand

The current implementation of real_path uses chdir() in order to resolve
symlinks.  Unfortunately this isn't thread-safe as chdir() affects a
process as a whole and not just an individual thread.  Instead perform
the symlink resolution by hand so that the calls to chdir() can be
removed, making real_path one step closer to being reentrant.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>