rerere: make sure it works even in a workdir attached to a young repository

The git-new-workdir script in contrib/ makes a new work tree by sharing
many subdirectories of the .git directory with the original repository.
When rerere.enabled is set in the original repository, but the user has
not encountered any conflicts yet, the original repository may not yet
have .git/rr-cache directory.

When rerere wants to run in a new work tree created from such a young
original repository, it fails to mkdir(2) .git/rr-cache that is a symlink
to a yet-to-be-created directory.

There are three possible approaches to this:

 - A naive solution is not to create a symlink in the git-new-workdir
   script to a directory the original does not have (yet).  This is not a
   solution, as we tend to lazily create subdirectories of .git/, and
   having rerere.enabled configuration set is a strong indication that the
   user _wants_ to have this lazy creation to happen;

 - We could always create .git/rr-cache upon repository creation.  This is
   tempting but will not help people with existing repositories.

 - Detect this case by seeing that mkdir(2) failed with EEXIST, checking
   that the path is a symlink, and try running mkdir(2) on the link
   target.

This patch solves the issue by doing the third one.

Strictly speaking, this is incomplete.  It does not attempt to handle
relative symbolic link that points into the original repository, but this
is good enough to help people who use contrib/workdir/git-new-workdir
script.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

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