gitg: add 0.2.6

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>
mask scm packages

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>
gitg: use xz

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>