support/download: fix git/svn corrupted cache

support/download: fix git/svn corrupted cache

Commit 54d3d94b6e3846447b5796ef8587b08b537cd348 ("support/download: print
command used for download") broke the git and svn download helpers, because
these helpers have invocations of the _git/_svn commands where the exact
output matters.

For example for git, this would result in:

    date: invalid date ‘GIT_DIR=.../dl/libyuv/git/.git git log -1 --pretty=format:%ci \n2019-04-12 17:48:45 +0000’
    Detected a corrupted git cache.
    Removing it and starting afresh.

Fix by splitting the _git function in two: _git and _plain_git.
The former echoes the command, and then calls the latter.
Most invocations use _git as before, but those cases where the output should
not be disturbed, directly call _plain_git.

For symmetry, all download helpers are aligned, even though only the git and
svn helpers were broken.

Fixes: #13631
Fixes:
    http://autobuild.buildroot.org/results/c2f/c2fcd4aa6660e3c2f9c6f85646ca7dfe0db56040/

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
[yann.morin.1998@free.fr: add bug report and autobuild failure]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>