Update .gitmodules to use shallow submodule clones

This change makes submodule checkouts shallow by default. This
significantly reduces the time needed to do a recursive checkout when
`--shallow-submodules` is not specified, such as when `x` is not being
used.

.gitmodules

@@ -1,34 +1,45 @@
 [submodule "src/doc/nomicon"]
 	path = src/doc/nomicon
 	url = https://github.com/rust-lang/nomicon.git
+	shallow = true
 [submodule "src/tools/cargo"]
 	path = src/tools/cargo
 	url = https://github.com/rust-lang/cargo.git
+	shallow = true
 [submodule "src/doc/reference"]
 	path = src/doc/reference
 	url = https://github.com/rust-lang/reference.git
+	shallow = true
 [submodule "src/doc/book"]
 	path = src/doc/book
 	url = https://github.com/rust-lang/book.git
+	shallow = true
 [submodule "src/doc/rust-by-example"]
 	path = src/doc/rust-by-example
 	url = https://github.com/rust-lang/rust-by-example.git
+	shallow = true
 [submodule "library/stdarch"]
 	path = library/stdarch
 	url = https://github.com/rust-lang/stdarch.git
+	shallow = true
 [submodule "src/doc/rustc-dev-guide"]
 	path = src/doc/rustc-dev-guide
 	url = https://github.com/rust-lang/rustc-dev-guide.git
+	shallow = true
 [submodule "src/doc/edition-guide"]
 	path = src/doc/edition-guide
 	url = https://github.com/rust-lang/edition-guide.git
+	shallow = true
 [submodule "src/llvm-project"]
 	path = src/llvm-project
 	url = https://github.com/rust-lang/llvm-project.git
 	branch = rustc/16.0-2023-06-05
+	shallow = true
 [submodule "src/doc/embedded-book"]
 	path = src/doc/embedded-book
 	url = https://github.com/rust-embedded/book.git
+	shallow = true
 [submodule "library/backtrace"]
 	path = library/backtrace
 	url = https://github.com/rust-lang/backtrace-rs.git
+	shallow = true