scan_path(): Improve handling of inaccessible directories

scan_path(): Improve handling of inaccessible directories

When scanning a tree containing inaccessible directories (e.g. '.ssh'
directories in users' homedirs, or repos with explicitly restricted access),
scan_path() currently causes three lines of "Permissions denied" errors to be
printed to the CGI error log per inaccessible directory:

  Error checking path /home/foo/.ssh: Permission denied (13)
  Error checking path /home/foo/.ssh/.git: Permission denied (13)
  Error opening directory /home/foo/.ssh: Permission denied (13)

This is a side-effect of calling is_git_dir(path) and
is_git_dir(fmt("%s/.git", path) _before_ we try to opendir(path).

By placing the opendir(path) before the two is_git_dir() calls, we reduce the
noise to a single line per inaccessible directory:

  Error opening directory /home/foo/.ssh: Permission denied (13)

Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Lars Hjemli <larsh@prediktor.no>