merge-ort: add implementation of both sides renaming identically

Implement rename/rename(1to1) handling, i.e. both sides of history
renaming a file but renaming the same way.  This code replaces the
following from merge-recurisve.c:

  * all the 1to1 code in process_renames()
  * the RENAME_ONE_FILE_TO_ONE case of process_entry()

Also, there is some shared code from merge-recursive.c for multiple
different rename cases which we will no longer need for this case (or
other rename cases):

  * handle_rename_normal()
  * setup_rename_conflict_info()

The consolidation of four separate codepaths into one is made possible
by a change in design: process_renames() tweaks the conflict_info
entries within opt->priv->paths such that process_entry() can then
handle all the non-rename conflict types (directory/file, modify/delete,
etc.) orthogonally.  This means we're much less likely to miss special
implementation of some kind of combination of conflict types (see
commits brought in by 66c62eaec6 ("Merge branch 'en/merge-tests'",
2020-11-18), especially commit ef52778708 ("merge tests: expect improved
directory/file conflict handling in ort", 2020-10-26) for more details).
That, together with letting worktree/index updating be handled
orthogonally in the merge_switch_to_result() function, dramatically
simplifies the code for various special rename cases.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

merge-ort.c

@@ -674,14 +674,30 @@ static int process_renames(struct merge_options *opt,
 		    !strcmp(oldpath, renames->queue[i+1]->one->path)) {
 			/* Handle rename/rename(1to2) or rename/rename(1to1) */
 			const char *pathnames[3];
+			struct version_info merged;
+			struct conflict_info *base, *side1, *side2;
 
 			pathnames[0] = oldpath;
 			pathnames[1] = newpath;
 			pathnames[2] = renames->queue[i+1]->two->path;
 
+			base = strmap_get(&opt->priv->paths, pathnames[0]);
+			side1 = strmap_get(&opt->priv->paths, pathnames[1]);
+			side2 = strmap_get(&opt->priv->paths, pathnames[2]);
+
+			VERIFY_CI(base);
+			VERIFY_CI(side1);
+			VERIFY_CI(side2);
+
 			if (!strcmp(pathnames[1], pathnames[2])) {
-				/* Both sides renamed the same way. */
-				die("Not yet implemented");
+				/* Both sides renamed the same way */
+				assert(side1 == side2);
+				memcpy(&side1->stages[0], &base->stages[0],
+				       sizeof(merged));
+				side1->filemask |= (1 << MERGE_BASE);
+				/* Mark base as resolved by removal */
+				base->merged.is_null = 1;
+				base->merged.clean = 1;
 
 				/* We handled both renames, i.e. i+1 handled */
 				i++;