download/git: fix basename for files inside tarballs

download/git: fix basename for files inside tarballs

Commit "6d938bcb52 download: git: introduce cache feature" introduced a
typo that makes the tarball to contain files without the package
basename:
$ tar -tvf good-a238b1dfcd825d47d834af3c5223417c8411d90d.tar.gz
-rw-r--r-- 0/0               8 2017-10-14 02:10 ./file

Historically, all tarballs are generated with the basename:
$ tar -tvf good-a238b1dfcd825d47d834af3c5223417c8411d90d.tar.gz
-rw-r--r-- 0/0               8 2017-10-14 02:10 good-a238b1dfcd825d47d834af3c5223417c8411d90d/file

The hashes in the tree were calculated with the basename.

In the most common scenario, after the download ends the tarball is
generated, the hash mismatches and the download mechanism falls back to
use the tarball from http://sources.buildroot.net .

The problem can be reproduced by forcing the download of any git package
PKG that has a hash file to check against:
$ make defconfig
$ ./utils/config --set-str BR2_BACKUP_SITE ""
$ BR2_DL_DIR=$(mktemp -d) make PKG-dirclean PKG-source

Fix the typo so the basename is really added to the files, that was
clearly the intention of the code.

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>