dgit: Postpone call to git_slurp_config

dgit: Postpone call to git_slurp_config

This is needed to fix #865863: we need to know what our operation is,
before we can decide whether to look for --local git config.

The code which now runs earlier is:
 * The messages about $dryrun_level (which is set only by
   the command line, and not by configuration - verified by
   searching for $dryrun_level).
 * Usage failure if @ARGV empty.  This is not affected by
   configuration.  (parseopts does the argument parsing and
   already runs before git_slurp_config.)
 * Extracting the $cmd from @ARGV.
 * Calling $pre_fn.  There is only one pre_* sub, which is
   pre_gbp_build.  It provides the default for $quilt_mode.
   $quilt_mode is indeed somewhat entangled with the git config,
   but this takes place in parseopts_late_defaults, which is
   called much later.

Therefore there is no functional change.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>