Indent the line with the source line.

def _set_line_indent(src, line, indent):
    '''
    Indent the line with the source line.
    '''
    if not indent:
        return line

    idt = []
    for c in src:
        if c not in ['\t', ' ']:
            break
        idt.append(c)

    return ''.join(idt) + line.lstrip()