git: avoid building `all` when not asked for
git: avoid building `all` when not asked for

For debugging purposes, it is nice to be able to simply remove the
`mingw-w64-git` package from the list of packages to build, and having
e.g. only the documentation packages being built.

Signed-off-by: Johannes Schindelin <53fb8db7833fca1e1746dd8592f61048d350f64c@gmx.de>

dscho/MINGW-packages,dscho/MINGW-packages,git-for-windows/MINGW-packages,dscho/MINGW-packages,dscho/MINGW-packages,git-for-windows/MINGW-packages,dscho/MINGW-packages,git-for-windows/MINGW-packages