feat(git): add guess_default_branch function

Author: Lee-W

commitizen/git.py

@@ -310,3 +310,10 @@ def _get_log_as_str_list(start: str | None, end: str, args: str) -> list[str]:
     if not c.out:
         return []
     return c.out.split(f"{delimiter}\n")
+
+
+def guess_default_branch() -> str:
+    c = cmd.run("git branch --format '%(refname:short)' --list master main")
+    if c.return_code != 0:
+        raise GitCommandError(c.err)
+    return c.out.strip()