Walk a path on the blob page from any commit the id names

With both an id and a path the blob page handed back the raw object
the id named, so a commit id gave the commit text itself, while the
plain page looked the path up in that commit's tree. A tag is peeled
to its commit on the way, and the README example of a pinned commit
now uses a form the request checks accept.

3 files changed, 29 insertions, 8 deletions

diff --git a/README.txt b/README.txt
index 5582164..d2eefea 100644
--- a/README.txt
+++ b/README.txt
@@ -192,7 +192,7 @@ Some worked examples, in the url= form.
     ?url=demo/log&h=next               log of the next branch
     ?url=demo/log&qt=author&q=alice    commits authored by alice
     ?url=demo/tree/src&h=v1.0          the src directory at tag v1.0
-    ?url=demo/commit&id=HEAD~3         one commit, pinned
+    ?url=demo/commit&id=v1.0           one commit, pinned to a tag
     ?url=demo/diff&id=main&id2=next    diff between two branches
     ?url=demo/plain/README.md          the raw bytes of one file
     ?url=demo/atom&h=main              the commit feed for a branch
diff --git a/source/ui-blob.c b/source/ui-blob.c
index 8407342..4105e48 100644
--- a/source/ui-blob.c
+++ b/source/ui-blob.c
@@ -181,17 +181,23 @@ void cgit_print_blob(const char *hex, char *path, const char *head, int file_onl
 			cgit_print_error_page(404, "Not Found", "Bad object id: %s", hex);
 			return;
 		}
-	} else {
-		if (repo_get_oid(the_repository, head, &oid)) {
-			cgit_print_error_page(404, "Not Found", "Bad object id: %s", head);
-			return;
-		}
-		peel_to_commit(&oid);
+	} else if (repo_get_oid(the_repository, head, &oid)) {
+		cgit_print_error_page(404, "Not Found", "Bad object id: %s", head);
+		return;
 	}
 
 	type = odb_read_object_info(the_repository->objects, &oid, &size);
 
-	if (!hex && type == OBJ_COMMIT && path) {
+	// A tag is peeled to its commit, so a path can be looked up in the
+	// tree whether the id or the head named the tag. The type is read
+	// first because peeling a blob would read it whole just to learn that
+	// it is one.
+	if (type == OBJ_TAG) {
+		peel_to_commit(&oid);
+		type = odb_read_object_info(the_repository->objects, &oid, &size);
+	}
+
+	if (type == OBJ_COMMIT && path) {
 		if (!find_path_oid(&oid, path, file_only)) {
 			cgit_print_error_page(404, "Not Found", "Path not found: %s", path);
 			return;
diff --git a/tests/t0303-robustness.sh b/tests/t0303-robustness.sh
index 73c2ca5..5bdb060 100755
--- a/tests/t0303-robustness.sh
+++ b/tests/t0303-robustness.sh
@@ -246,6 +246,21 @@ test_expect_success 'the blob page reads a file through an annotated tag' '
 	test_cmp want body
 '
 
+# The plain page walks a path out of whatever commit the id names, and the
+# blob page used to answer such a request with the raw commit object.
+test_expect_success 'the blob page walks a path out of the commit or tag the id names' '
+	tip=$(git -C repos/rob rev-parse HEAD) &&
+	robq "url=rob/blob/file-1&id=$tip" >tmp &&
+	strip_headers <tmp >body &&
+	test_cmp want body &&
+	tag=$(git -C repos/rob rev-parse ann) &&
+	robq "url=rob/blob/file-1&id=$tag" >tmp &&
+	strip_headers <tmp >body &&
+	test_cmp want body &&
+	robq "url=rob/blob/missing&id=$tip" >tmp &&
+	grep "^Status: 404" tmp
+'
+
 test_expect_success 'the commit page shows a message whole when text follows its trailers' '
 	(
 		cd repos/rob &&