To be clear - This is not a git diff problem or question
I have two locally checked out repos - that share a common quasi common heritage.
Both are under git control (but come from two separate remote servers) - they have many things in common.
What I would like to do is a normal gnu diff -Naur type comparison between the two directory structures.
Normally you would use --exclude=PATTERN options for gnu-diff to exclude things. For example you might --exclude=.git to ignore the two different .git directories
What I want instead is an option to make gnu-diff .gitignore aware
in effect, as gnu diff traverses the directory structure and comes across a .gitignore file follow the GIT rules and ignore accordingly.
Sort of an option like this:
diff -Naur --honor-git-ignore-files DIR1 DIR2
Since there does not seem to be a default linux diff option like the one you mention, you could:
git ls-files --exclude-standard) in both Git repos,