[PATCH 1/1] difftool --no-index: error out on --dir-diff (and don't crash)

["Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com>]

From: Johannes Schindelin <johannes.schindelin@gmx.de>

In `--no-index` mode, we now no longer require a worktree nor a
repository. But some code paths in `difftool` expect those to be
present.

The most notable such code path is the `--dir-diff` one: we use the
existing checkout machinery to copy the files, and that machinery looks
up replacement refs, looks at alternate ODBs, wants to use the worktree
path, etc.

Rather than running into segmentation faults, let's die with an
informative error message.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 builtin/difftool.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/builtin/difftool.c b/builtin/difftool.c
index 4fff1e83f9..5704a76088 100644
--- a/builtin/difftool.c
+++ b/builtin/difftool.c
@@ -735,7 +735,8 @@ int cmd_difftool(int argc, const char **argv, const char *prefix)
 		setup_work_tree();
 		setenv(GIT_DIR_ENVIRONMENT, absolute_path(get_git_dir()), 1);
 		setenv(GIT_WORK_TREE_ENVIRONMENT, absolute_path(get_git_work_tree()), 1);
-	}
+	} else if (dir_diff)
+		die(_("--dir-diff is incompatible with --no-index"));
 
 	if (use_gui_tool && diff_gui_tool && *diff_gui_tool)
 		setenv("GIT_DIFF_TOOL", diff_gui_tool, 1);
-- 
gitgitgadget