Git output send on stderr

[silkeh]

Stderr of all commands should be redirected to stdout since #63. This is not the case for Git. For example:

> sudo solbuild build > /tmp/element.log
 ✓ > Using default profile name=unstable-x86_64
Cloning into '/var/lib/solbuild/sources/git/github.com/vector-im/element-desktop.git'...
remote: Enumerating objects: 188, done.
remote: Counting objects: 100% (188/188), done.
remote: Compressing objects: 100% (164/164), done.
remote: Total 188 (delta 30), reused 111 (delta 13), pack-reused 0
Receiving objects: 100% (188/188), 3.07 MiB | 28.33 MiB/s, done.
Resolving deltas: 100% (30/30), done.
Note: switching to '56a4f7df41a72bba88f693ce6e9464161e73306c'.
[etc]

[silkeh]

Resolved in #82.