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.go

@@ -258,6 +258,21 @@ func (e *gitCommandError) Unwrap() error {
 	return e.err
 }
 
+func gitCommandEnv(authEnv []string) []string {
+	baseEnv := os.Environ()
+	env := make([]string, 0, len(baseEnv)+len(authEnv)+3)
+	for _, value := range baseEnv {
+		if strings.HasPrefix(value, "LC_ALL=") ||
+			strings.HasPrefix(value, "LANG=") ||
+			strings.HasPrefix(value, "LANGUAGE=") {
+			continue
+		}
+		env = append(env, value)
+	}
+	env = append(env, authEnv...)
+	return append(env, "LC_ALL=C", "LANG=C", "LANGUAGE=C")
+}
+
 func runGitCommand(dir string, auth *AuthMethod, extraConfigs []string, args ...string) (string, error) {
 	authConfigs, authEnv, cleanup, err := prepareCLIAuth(auth)
 	if err != nil {
@@ -278,7 +293,7 @@ func runGitCommand(dir string, auth *AuthMethod, extraConfigs []string, args ...
 	if dir != "" {
 		cmd.Dir = dir
 	}
-	cmd.Env = append(os.Environ(), authEnv...)
+	cmd.Env = gitCommandEnv(authEnv)
 
 	var stdout bytes.Buffer
 	var stderr bytes.Buffer