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. In order to
get git to print the correct position of the error when there is an
unwanted tab in the indent following a space we need to turn off
`space-before-tab` otherwise the space will be considered to be an error
instead of the tab.

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