* guix/build/download.scm
(string-pad-middle, store-url-abbreviation, store-path-abbreviation):
New procedures.
(progress-proc): Add #:abbreviation parameter and use it. Generate a
better indeterminate progress string.
* guix/scripts/substitute.scm (assert-valid-narinfo): Add newlines to output.
(process-substitution): Use byte-count->string and store-path-abbreviation.
Co-authored-by: Ludovic Courtès <ludo@gnu.org>