bug#30113: SVN checkouts without descriptive file names

[Leo Famulari]

Working on core-updates, I noticed this in the list of substitutes to be
downloaded:

[...]
/gnu/store/gyzn9z9mvhcmkmgdqz9b0vqanrz7v7qr-svn-checkout
/gnu/store/414npvzajl508j1kaji1piy52byr7b1y-svn-checkout
/gnu/store/zj29xvq1nw5zra11h0y7vq5lycv205lw-svn-checkout
/gnu/store/3bh3fp1kmhmk27dngif0x3np1rrnxhzn-svn-checkout
/gnu/store/jhvrk725xk1wwab2byvx4dziwjvhpd0y-svn-checkout
/gnu/store/115icxjrw0677fnyf3gl0nihqvbrm5lb-svn-checkout
/gnu/store/2kbdl6cyr2b7ibscpm768rfnxp7a7hyg-svn-checkout
/gnu/store/yfs603a9a4h8zzkdvmyz3krkj12rxhwf-svn-checkout
/gnu/store/w7pdh5ng3gzc2a4smfkyr2wp7jdhfhpp-svn-checkout
/gnu/store/n3p72br441m2098bfcjlvpn8807hq97p-svn-checkout
/gnu/store/aczcl41x9qh6nvlzdplibh4scg3cihbr-svn-checkout
/gnu/store/77c0hzkb7n6pid09fymmfrjjzkaf10yi-svn-checkout
/gnu/store/j8k2p0rknzkgbzmy4scxlpvrh2g2zl5q-svn-checkout
/gnu/store/kqch1yqy0zj58qlrk2p9kqnrwn57a3gl-svn-checkout
/gnu/store/nk2rkdq8x6vwkvn0h2sfrbmg0sjzvias-svn-checkout
/gnu/store/vwvqvl1f79q34r4gb4wsli36yzcqg4j9-svn-checkout
/gnu/store/f7hhc7wcyxvvn9m9km1j6ghamqnlj4id-svn-checkout
/gnu/store/23wba6lz101s3cs8vl25r28np8x7sbcs-texlive-latex-hyperref-6.84a2.tar.gz
/gnu/store/rgyb8rhpbmb2c4wv7k512z82x82aik74-svn-checkout
/gnu/store/i5yp4xkqkjm3v1cj5ffjgq42h4nvq435-svn-checkout
/gnu/store/hz7f8b8rc9zyxxpyywbrsgz1spm0rwr9-svn-checkout
[...]

We should track these down and give them some real file names
eventually.