dgit: Add many pre_* to call no_local_git_cfg

dgit: Add many pre_* to call no_local_git_cfg

For each operation which can meaningfully be run outside a git tree,
arrange to call no_local_git_cfg and thus avoid running
  git config --local

There is one slight infelicity: some subcommands (notably
archive-api-query) could in theory be run within a git tree and expect
that git tree to influence their output.  However, this seems
unlikely.  In fact, I think there are probably only in-tree callers
and the in-tree callers do not do this.

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