Rust dependency on git

[Ivan Petkov]

Hi Guix,

The latest stable rust (version 1.35.0) appears to require git as a
native-input since the tidy test (which checks for code formatting in the rust
repo) fails because it cannot run `git ls-files`.

I saw a comment in the rust package definition that git changes too frequently
to be kept as an input[1]. Is anyone aware if this is still a problem?
I'd rather avoid having to patch the test suite tools to ignore this test
if it is as simple as adding "git" as a native input.

cc Marius who added this line according to the history.

--Ivan

[1]: 
https://git.savannah.gnu.org/cgit/guix.git/tree/gnu/packages/rust.scm?id=e26d628b0fabf5a0aa7c4164a9558c66c61e02ab#n467