Add contrib/credentials/netrc with GPG support

This credential helper support multiple files, returning the first
one that matches.  It checks file permissions and owner.  For *.gpg
files, it will run GPG to decrypt the file.

Signed-off-by: Ted Zlatanov <tzz@lifelogs.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
[SQUASH???] minimum fix-ups

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge branch 'tz/credential-authinfo' into pu

* tz/credential-authinfo:
  [SQUASH???] minimum fix-ups
  Add contrib/credentials/netrc with GPG support