sequencer: ensure labels that are object IDs are rewritten

When writing the todo script for --rebase-merges, we try to find a label
for certain commits.  If the label ends up being a valid object ID, such
as when we merge a detached commit, we want to rewrite it so it is no
longer a valid object ID.

However, the code path that does this checks for its length to be
equivalent to GIT_SHA1_RAWSZ, which isn't correct, since what we are
reading is a hex object ID.  Instead, check for the length being
equivalent to that of a hex object ID.  Use the_hash_algo so this code
works regardless of the hash size.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: bk2204

sequencer.c

@@ -3534,7 +3534,7 @@ static const char *label_oid(struct object_id *oid, const char *label,
 				p[i] = save;
 			}
 		}
-	} else if (((len = strlen(label)) == GIT_SHA1_RAWSZ &&
+	} else if (((len = strlen(label)) == the_hash_algo->hexsz &&
 		    !get_oid_hex(label, &dummy)) ||
 		   (len == 1 && *label == '#') ||
 		   hashmap_get_from_hash(&state->labels,

t/t3430-rebase-merges.sh

@@ -70,6 +70,7 @@ test_expect_success 'create completely different structure' '
 	merge -C H second
 	merge onebranch # Merge the topic branch '\''onebranch'\''
 	EOF
+	cp script-from-scratch script-from-scratch-orig &&
 	test_config sequence.editor \""$PWD"/replace-editor.sh\" &&
 	test_tick &&
 	git rebase -i -r A &&
@@ -241,4 +242,20 @@ test_expect_success 'refuse to merge ancestors of HEAD' '
 	test_cmp_rev HEAD $before
 '
 
+test_expect_success 'labels that are object IDs are rewritten' '
+	git checkout -b third B &&
+	test_tick &&
+	test_commit I &&
+	third=$(git rev-parse HEAD) &&
+	git checkout -b labels master &&
+	git merge --no-commit third &&
+	test_tick &&
+	git commit -m "Merge commit '\''$third'\'' into labels" &&
+	cp script-from-scratch-orig script-from-scratch &&
+	test_config sequence.editor \""$PWD"/replace-editor.sh\" &&
+	test_tick &&
+	git rebase -i -r A &&
+	! grep "^label $third$" .git/ORIGINAL-TODO
+'
+
 test_done