Fix locale-dependent git repo detection (#1099)

Fixes #1089

## Root cause
`tea clone` initializes the command context by probing the current directory with `git rev-parse --show-toplevel`. `classifyRepoError` recognizes the "not a git repository" case by matching the English stderr text. When git emits localized errors (for example with `LANG=de_DE.UTF-8`), that match fails, and the otherwise harmless "no repo here" error is returned as fatal before cloning.

## Fix
Pin the locale of git subprocesses to `C` by overriding `LC_ALL`, `LANG`, and `LANGUAGE` in `runGitCommand`. This keeps all git output stable for parsing while leaving the parent process locale untouched.

## Tests
Added a regression test using a fake git executable that emits a German "not a repository" error unless the locale is forced to `C`, and verifies `ErrRepositoryNotExists` is returned.

Reviewed-on: https://gitea.com/gitea/tea/pulls/1099
Reviewed-by: Zettat123 <39446+zettat123@noreply.gitea.com>

modules/git/cli_backend_test.go

@@ -0,0 +1,40 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package git
+
+import (
+	"os"
+	"path/filepath"
+	"runtime"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestOpenClassifiesLocalizedNotRepoError(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skip("test uses a POSIX shell script as fake git")
+	}
+
+	fakeGitDir := t.TempDir()
+	fakeGit := filepath.Join(fakeGitDir, "git")
+	script := `#!/bin/sh
+locale="${LC_ALL:-${LANG:-C}}"
+if [ "$locale" = "C" ]; then
+	echo "fatal: not a git repository (or any of the parent directories): .git" >&2
+else
+	echo "fatal: Kein Git-Repository (oder irgendein Elternverzeichnis bis zum Einhangepunkt /)" >&2
+fi
+exit 128
+`
+	require.NoError(t, os.WriteFile(fakeGit, []byte(script), 0o755))
+
+	t.Setenv("PATH", fakeGitDir+string(os.PathListSeparator)+os.Getenv("PATH"))
+	t.Setenv("LC_ALL", "de_DE.UTF-8")
+	t.Setenv("LANG", "de_DE.UTF-8")
+	t.Setenv("LANGUAGE", "de_DE:de")
+
+	_, err := (cliBackend{}).Open(t.TempDir())
+	require.ErrorIs(t, err, ErrRepositoryNotExists)
+}