git-svn: search --authors-prog in PATH too

git-svn: search --authors-prog in PATH too

In 36db1eddf9 ("git-svn: add --authors-prog option", 2009-05-14) the path
to authors-prog was made absolute because git-svn changes the current
directory in some situations. This makes sense if the program is part of
the repository but prevents searching via $PATH.

The old behaviour is still retained, but if the file does not exists, then
authors-prog is searched for in $PATH as any other command.

Signed-off-by: Andreas Heiduk <asheiduk@gmail.com>
Signed-off-by: Eric Wong <e@80x24.org>