dgit-maint-merge(7): Configure --merge-mode for gbp-import-orig(1)

For 3.0 (quilt) source packages, gbp-import-orig(1) defaults to
--merge-mode=replace, which does the wrong thing for patches-applied
repositories (see gbp-import-orig(1)).

Note that we do override this when importing the first upstream
version.  This is needed because git-merge(1) will refuse to merge
unrelated histories.

Thanks to Johannes Schauer for reporting the problem.

Signed-off-by: Sean Whitton <spwhitton@spwhitton.name>