add .gitattributes rules for rust files

Setting `diff=rust` will make git show the current function name in the
hunk headers.

While delta currently removes git's whitespace error markup from diffs
adding the whitespace rules will mean pre-commit hooks that run `git
diff --check` will reject changes with whitespace errors.

The whitespace rule could perhaps be applied more widely but the
Makefile and example gitcofig have leading tabs so I restricted them to
rust files for now.

.gitattributes

@@ -1,2 +1,3 @@
 *.Makefile linguist-language=Makefile
 etc/performance/* linguist-vendored
+*.rs diff=rust whitespace=tab-in-indent,trailing-space,space-before-tab eol=lf