progressbar: fix behaviour under redirection

[jdtournier]

Minor issue reported by @dchristiaens , but fixes a problem left over from #1586. This only influences the progressbar updates when redirected away from the terminal.

Example for a simple mrconvert DICOM to nii conversion:

mrconvert 3dfl/ anat.nii 2> log.txt

Content of log.txt using current master:

 done
============================================]
=====================]

Content of log.txt using this PR:

mrconvert: scanning DICOM folder "3dfl/"... done
mrconvert: reading DICOM series "fl3D_1x1x1_tra"... [==================================================]
mrconvert: copying from "XXXXXXXX...ri vol) [MR] fl3D_1x1x1_tra" to "anat.nii"... [==================================================]

[dchristiaens]

Thanks @jdtournier, this fixes the problem with the scripts I was running. I also tested on macOS and ZSH to be sure, all fine :)