Redundant cloning of git submodules

[MagaTailor]

When building from git source, the build system makes a deep clone of git submodules, even those, like jemalloc or llvm, that had been disabled with configure options. (--disable-jemalloc and --llvm-root respectively)

Not downloading a couple hundred megabytes of data spread over tens of thousands of small files is especially important on flash devices (and slow connections).

[Aatch]

At the very least it should probably do a shallower clone of those repos. As best I can tell, you should be able to just pass --depth 1 to git submodule update just fine.

[MagaTailor]

Yep, that would be a great start. However the gist of this report is to completely forego downloading the source trees that are never going to be used.

[MagaTailor]

@japaric I've just tested your patch, having done a rust shallow clone first and llvm is not cloned at all but jemalloc still is. Still, a huge improvement; thanks!

I also paid attention to the amount of data downloaded by fully cloning other modules, like compiler-rt, liblibc, etc. and it seems bootstrapping from a source snapshot is still the fastest option, requiring a 50% smaller download.