[gitmodules] Ignore dirty submodules (#746)

Main motivation for this is to make switching between branches faster:
Down from 15 seconds to light-speed.

For things like `git status` and `git diff` I've figured out how to make
them fast without this change: use the `--ignore-submodules` flags. But
for `git checkout` I haven't found an equivalent.

I guess the underlying problem is that the submodules (XRT, etc.) get
dirtied after cloning, and somehow this makes more work for `git`.
Untracked files like `src/runtime_src/core/common/api/version.h` etc
appear. I'm not 100% sure of this.

If someone has an alternative suggestion to getting lighting fast `git
checkout` without this PR: please let me know.

.gitmodules

@@ -1,25 +1,31 @@
 [submodule "third_party/XRT"]
-	path = third_party/XRT
-	url = https://github.com/nod-ai/XRT.git
-	shallow = true
-	branch = iree-amd-aie-patches
+  path = third_party/XRT
+  url = https://github.com/nod-ai/XRT.git
+  shallow = true
+  branch = iree-amd-aie-patches
+  ignore = dirty
 [submodule "third_party/mlir-air"]
-	path = third_party/mlir-air
-	url = https://github.com/nod-ai/mlir-air.git
-	shallow = true
+  path = third_party/mlir-air
+  url = https://github.com/nod-ai/mlir-air.git
+  shallow = true
+  ignore = dirty
 [submodule "third_party/aie-rt"]
-	path = third_party/aie-rt
-	url = https://github.com/Xilinx/aie-rt.git
-	shallow = true
+  path = third_party/aie-rt
+  url = https://github.com/Xilinx/aie-rt.git
+  shallow = true
+  ignore = dirty
 [submodule "third_party/bootgen"]
-	path = third_party/bootgen
-	url = https://github.com/Xilinx/bootgen.git
-	shallow = true
+  path = third_party/bootgen
+  url = https://github.com/Xilinx/bootgen.git
+  shallow = true
+  ignore = dirty
 [submodule "third_party/openssl"]
-	path = third_party/openssl
-	url = https://github.com/viaduck/openssl-cmake.git
-	shallow = true
+  path = third_party/openssl
+  url = https://github.com/viaduck/openssl-cmake.git
+  shallow = true
+  ignore = dirty
 [submodule "third_party/iree"]
-	path = third_party/iree
-	url = https://github.com/iree-org/iree.git
-	shallow = true
+  path = third_party/iree
+  url = https://github.com/iree-org/iree.git
+  shallow = true
+  ignore = dirty