dgit, git-debrebase: Properly make patches with nasty .gitignores.

Specifically:

 * When running git status --porcelain, pass --ignored so it
   lists ignored files, and handle the ! output that it then
   produces.

 * Add missing -f to a few git add invocations.

I have done some greps ('git.*add', 'git.*status', 'porcelain', etc.)
to try to find other missed cases and none turned up.

git diff is OK because normally we pass two treeish arguments, in
which case the ignores are ignored by git diff.  When we are asking it
to look at the working tree, we are expecting it to ignore untracked
files (whether ignored or not), and diff tracked ones, which is what
it does.

Closes:#903130.

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