Fix progress bar width calculation
Using current JHBuild I get the following error when bootstrapping on macOS:
[... removed for readability ...]
frontends/terminal.py", line 151, in display_status_line
progress_bar = '[' + (num_hashes * '=') + ((width - num_hashes) * '-') + ']'
TypeError: can't multiply sequence by non-int of type 'float'
This MR fixes that.