Merge branch 'jk/diff-submodule-diff-inline'

The "git diff --submodule={short,log}" mechanism has been enhanced
to allow "--submodule=diff" to show the patch between the submodule
commits bound to the superproject.

* jk/diff-submodule-diff-inline:
  diff: teach diff to display submodule difference with an inline diff
  submodule: refactor show_submodule_summary with helper function
  submodule: convert show_submodule_summary to use struct object_id *
  allow do_submodule_path to work even if submodule isn't checked out
  diff: prepare for additional submodule formats
  graph: add support for --line-prefix on all graph-aware output
  diff.c: remove output_prefix_length field
  cache: add empty_tree_oid object and helper function

path.c

@@ -6,6 +6,7 @@
 #include "string-list.h"
 #include "dir.h"
 #include "worktree.h"
+#include "submodule-config.h"
 
 static int get_st_mode_bits(const char *path, int *mode)
 {
@@ -468,12 +469,16 @@ const char *worktree_git_path(const struct worktree *wt, const char *fmt, ...)
 	return pathname->buf;
 }
 
-static void do_submodule_path(struct strbuf *buf, const char *path,
-			      const char *fmt, va_list args)
+/* Returns 0 on success, negative on failure. */
+#define SUBMODULE_PATH_ERR_NOT_CONFIGURED -1
+static int do_submodule_path(struct strbuf *buf, const char *path,
+			     const char *fmt, va_list args)
 {
 	const char *git_dir;
 	struct strbuf git_submodule_common_dir = STRBUF_INIT;
 	struct strbuf git_submodule_dir = STRBUF_INIT;
+	const struct submodule *sub;
+	int err = 0;
 
 	strbuf_addstr(buf, path);
 	strbuf_complete(buf, '/');
@@ -484,6 +489,17 @@ static void do_submodule_path(struct strbuf *buf, const char *path,
 		strbuf_reset(buf);
 		strbuf_addstr(buf, git_dir);
 	}
+	if (!is_git_directory(buf->buf)) {
+		gitmodules_config();
+		sub = submodule_from_path(null_sha1, path);
+		if (!sub) {
+			err = SUBMODULE_PATH_ERR_NOT_CONFIGURED;
+			goto cleanup;
+		}
+		strbuf_reset(buf);
+		strbuf_git_path(buf, "%s/%s", "modules", sub->name);
+	}
+
 	strbuf_addch(buf, '/');
 	strbuf_addbuf(&git_submodule_dir, buf);
 
@@ -494,27 +510,38 @@ static void do_submodule_path(struct strbuf *buf, const char *path,
 
 	strbuf_cleanup_path(buf);
 
+cleanup:
 	strbuf_release(&git_submodule_dir);
 	strbuf_release(&git_submodule_common_dir);
+
+	return err;
 }
 
 char *git_pathdup_submodule(const char *path, const char *fmt, ...)
 {
+	int err;
 	va_list args;
 	struct strbuf buf = STRBUF_INIT;
 	va_start(args, fmt);
-	do_submodule_path(&buf, path, fmt, args);
+	err = do_submodule_path(&buf, path, fmt, args);
 	va_end(args);
+	if (err) {
+		strbuf_release(&buf);
+		return NULL;
+	}
 	return strbuf_detach(&buf, NULL);
 }
 
-void strbuf_git_path_submodule(struct strbuf *buf, const char *path,
-			       const char *fmt, ...)
+int strbuf_git_path_submodule(struct strbuf *buf, const char *path,
+			      const char *fmt, ...)
 {
+	int err;
 	va_list args;
 	va_start(args, fmt);
-	do_submodule_path(buf, path, fmt, args);
+	err = do_submodule_path(buf, path, fmt, args);
 	va_end(args);
+
+	return err;
 }
 
 static void do_git_common_path(struct strbuf *buf,