merge-recursive: do not barf on "to be removed" entries.

When update-trees::threeway_merge() decides that a path that
exists in the current index (and HEAD) is to be removed, it
leaves a stage 0 entry whose mode bits are set to 0.  The code
mistook it as "this stage wants the blob here", and proceeded
to call update_file_flags() which ended up trying to put the
mode=0 entry in the index, got very confused, and ended up
barfing with "do not know what to do with 000000".

Since threeway_merge() does not handle case #10 (one side
removes while the other side does not do anything), this is not
a problem while we refuse to merge branches that have D/F
conflicts, but when we start resolving them, we would need to be
able to remove cache entries, and at that point it starts to
matter.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

merge-recursive.c

@@ -1018,9 +1018,9 @@ static int process_renames(struct path_list *a_renames,
 	return clean_merge;
 }
 
-static unsigned char *has_sha(const unsigned char *sha)
+static unsigned char *stage_sha(const unsigned char *sha, unsigned mode)
 {
-	return is_null_sha1(sha) ? NULL: (unsigned char *)sha;
+	return (is_null_sha1(sha) || mode == 0) ? NULL: (unsigned char *)sha;
 }
 
 /* Per entry merge function */
@@ -1033,12 +1033,12 @@ static int process_entry(const char *path, struct stage_data *entry,
 	print_index_entry("\tpath: ", entry);
 	*/
 	int clean_merge = 1;
-	unsigned char *o_sha = has_sha(entry->stages[1].sha);
-	unsigned char *a_sha = has_sha(entry->stages[2].sha);
-	unsigned char *b_sha = has_sha(entry->stages[3].sha);
 	unsigned o_mode = entry->stages[1].mode;
 	unsigned a_mode = entry->stages[2].mode;
 	unsigned b_mode = entry->stages[3].mode;
+	unsigned char *o_sha = stage_sha(entry->stages[1].sha, o_mode);
+	unsigned char *a_sha = stage_sha(entry->stages[2].sha, a_mode);
+	unsigned char *b_sha = stage_sha(entry->stages[3].sha, b_mode);
 
 	if (o_sha && (!a_sha || !b_sha)) {
 		/* Case A: Deleted in one */
@@ -1139,6 +1139,12 @@ static int process_entry(const char *path, struct stage_data *entry,
 				update_file_flags(mfi.sha, mfi.mode, path,
 					      0 /* update_cache */, 1 /* update_working_directory */);
 		}
+	} else if (!o_sha && !a_sha && !b_sha) {
+		/*
+		 * this entry was deleted altogether. a_mode == 0 means
+		 * we had that path and want to actively remove it.
+		 */
+		remove_file(1, path, !a_mode);
 	} else
 		die("Fatal merge failure, shouldn't happen.");