git annex diffdriver -- cmd --opts --
This is an external git diff driver shim. Normally, when using git diff with an external diff driver, the symlinks to annexed files are not set up right, so the external diff driver cannot read them in order to perform smart diffing of their contents. This command works around the problem, by passing the fixed up files to the real external diff driver.
To use this, you will need to have installed some git external diff driver command. This is not the regular diff command; it takes a git-specific input. See git's documentation of GIT_EXTERNAL_DIFF and gitattributes(5)'s documentation of external diff drivers.
Configure git to use "git-annex diffdriver -- cmd params --" as the external diff driver, where cmd is the external diff driver you want it to run, and params are any extra parameters to pass to it. Note the trailing "--", which is required.
For example, set GIT_EXTERNAL_DIFF=git-annex diffdriver -- j-c-diff --
Joey Hess <email@example.com>