So here's my attempt at fixing this.

The thing I was missing is that init_revisions() calls diff_setup(), which
sets the xdl options.  It's therefore necessary to have the
diff_indent_heuristic flag set before calling init_revisions().

A naive way to get the indentHeuristic config option respected in the
diff-* plumbing commands is to make them use the git_diff_heuristic_config()
callback right at the start of their main cmd functions.

But I did not like that for two reasons:

* It would make these commands invoke git_config() twice.

* It doesn't avoid the problem if/when someone creates a new diff-something
  plumbing command, and forgets to set the diff_indent_heuristic flag before
  calling init_revisions().

So instead I chose to make the indentHeuristic option part of diff's basic
configuration, and in each of the diff plumbing commands I moved the call to
git_config() before the call to init_revisions().

This still doesn't really future-proof things for possible new diff plumbing
commands, because someone could still invoke init_revisions() before setting
up diff's basic configuration.  But I don't see an obvious way of ensuring
that the diff_indent_heuristic flag is respected regardless of when
diff_setup() is invoked.

		M.

Marc Branchaud (2):
  Make the indent heuristic part of diff's basic configuration.
  Have the diff-* builtins configure diff before initializing revisions.

 builtin/diff-files.c | 2 +-
 builtin/diff-index.c | 2 +-
 builtin/diff-tree.c  | 2 +-
 diff.c               | 6 +++---
 4 files changed, 6 insertions(+), 6 deletions(-)