Fix TypeError when using the --origin option

Encode the `name` parameter to bytes (using the utf8 codec).

This fixes the `TypeError` in subsequent concatenations in `get_branch`:

```
Traceback (most recent call last):
  # stack omitted for brevity
  File "C:\Dev\git-migration\fast-export\hg2git.py", line 73, in get_branch
    return origin_name + b'/' + name
TypeError: can only concatenate str (not "bytes") to str
```

The conversion is done unconditionally since the passed
parameter is currently always of type `str`.

hg2git.py

@@ -27,7 +27,7 @@ def set_default_branch(name):
 
 def set_origin_name(name):
   global origin_name
-  origin_name = name
+  origin_name = name.encode('utf8')
 
 def setup_repo(url):
   try:

t/set_origin.expected

@@ -0,0 +1,42 @@
+blob
+mark :1
+data 5
+zero
+
+reset refs/heads/prefix/master
+commit refs/heads/prefix/master
+mark :2
+author H G Wells <wells@example.com> 1679014800 +0000
+committer H G Wells <wells@example.com> 1679014800 +0000
+data 5
+zero
+M 100644 :1 content
+
+blob
+mark :3
+data 8
+branch1
+
+commit refs/heads/prefix/branch1
+mark :4
+author H G Wells <wells@example.com> 1679018400 +0000
+committer H G Wells <wells@example.com> 1679018400 +0000
+data 29
+Added file in branch branch1
+from :2
+M 100644 :3 b8486c4feca589a4237a1ee428322d7109ede12e
+
+blob
+mark :5
+data 8
+branch2
+
+commit refs/heads/prefix/branch2
+mark :6
+author H G Wells <wells@example.com> 1679022000 +0000
+committer H G Wells <wells@example.com> 1679022000 +0000
+data 29
+Added file in branch branch2
+from :4
+M 100644 :5 fe786baee0d76603092c25609f2967b9c28a2cf2
+

t/set_origin.t

@@ -0,0 +1,59 @@
+#!/bin/bash
+#
+# Copyright (c) 2023 Felipe Contreras
+# Copyright (c) 2025 Günther Nußmüller
+#
+
+test_description='Set origin tests'
+
+. "${SHARNESS_TEST_SRCDIR-$(dirname "$0")/sharness}"/sharness.sh || exit 1
+
+check() {
+	git -C "$1" fast-export --all > actual
+	test_cmp "$SHARNESS_TEST_DIRECTORY"/set_origin.expected actual
+}
+
+git_clone() {
+	(
+	git init -q "$2" &&
+	cd "$2" &&
+	git config core.ignoreCase false &&
+	hg-fast-export.sh --repo "../$1" --origin "$3"
+	)
+}
+
+setup() {
+	cat > "$HOME"/.hgrc <<-EOF
+	[ui]
+	username = H G Wells <wells@example.com>
+	EOF
+}
+
+make-branch() {
+	hg branch "$1"
+	FILE=$(echo "$1" | sha1sum | cut -d " " -f 1)
+	echo "$1" > $FILE
+	hg add $FILE
+	hg commit -d "2023-03-17 $2:00Z" -m "Added file in branch $1"
+}
+
+setup
+
+test_expect_success 'basic' '
+	test_when_finished "rm -rf hgrepo gitrepo" &&
+
+	(
+	hg init hgrepo &&
+	cd hgrepo &&
+	echo zero > content &&
+	hg add content &&
+	hg commit -m zero -d "2023-03-17 01:00Z" &&
+	make-branch branch1 02 &&
+	make-branch branch2 03
+	) &&
+
+	git_clone hgrepo gitrepo prefix &&
+	check gitrepo
+'
+
+test_done