git-svn: add --authors-prog option

git-svn: add --authors-prog option

Add a new option, --authors-prog, to git-svn that allows a more flexible
alternative (or supplement) to --authors-file.  This allows more
advanced username operations than the authors file will allow.  For
example, one may look up Subversion users via LDAP, or may generate the
name and email address from the Subversion username.

Notes:

* If both --authors-name and --authors-prog are given, the former is
  tried first, falling back to the later.

* The program is called once per unique SVN username, and the result is
  cached.

* The command-line argument must be the path to a program, not a generic
  shell command line.  The absolute path to this program is taken at
  startup since the git-svn script changes directory during operation.

* The option is not enabled for `git svn log'.

[ew: fixed case where neither --authors-(name|prog) were defined]
Signed-off-by: Mark Lodato <lodatom@gmail.com>
Acked-by: Eric Wong <normalperson@yhbt.net>