Fix tests in //api_proto on Windows

Fix the Starlark rule generating
//api_proto:api.gen.pb.go_checkshtest so it works
on Windows, where a runfiles tree is not
available.

To fix, the generated shell script now uses the
Bash runfiles library to locate the files to
compare.

See bazelbuild/bazel#5508

build/build_defs.bzl

@@ -58,12 +58,46 @@ def genfile_check_test(src, gen):
     native.genrule(
         name = src + "_checksh",
         outs = [src + "_check.sh"],
-        cmd = "echo 'diff $$@' > $@",
+        cmd = r"""cat >$@ <<'eof'
+#!/bin/bash
+# Script generated by @com_github_bazelbuild_buildtools//build:build_defs.bzl
+
+# --- begin runfiles.bash initialization ---
+# Copy-pasted from Bazel's Bash runfiles library (tools/bash/runfiles/runfiles.bash).
+set -euo pipefail
+if [[ ! -d "$${RUNFILES_DIR:-/dev/null}" && ! -f "$${RUNFILES_MANIFEST_FILE:-/dev/null}" ]]; then
+  if [[ -f "$$0.runfiles_manifest" ]]; then
+    export RUNFILES_MANIFEST_FILE="$$0.runfiles_manifest"
+  elif [[ -f "$$0.runfiles/MANIFEST" ]]; then
+    export RUNFILES_MANIFEST_FILE="$$0.runfiles/MANIFEST"
+  elif [[ -f "$$0.runfiles/bazel_tools/tools/bash/runfiles/runfiles.bash" ]]; then
+    export RUNFILES_DIR="$$0.runfiles"
+  fi
+fi
+if [[ -f "$${RUNFILES_DIR:-/dev/null}/bazel_tools/tools/bash/runfiles/runfiles.bash" ]]; then
+  source "$${RUNFILES_DIR}/bazel_tools/tools/bash/runfiles/runfiles.bash"
+elif [[ -f "$${RUNFILES_MANIFEST_FILE:-/dev/null}" ]]; then
+  source "$$(grep -m1 "^bazel_tools/tools/bash/runfiles/runfiles.bash " \
+            "$$RUNFILES_MANIFEST_FILE" | cut -d ' ' -f 2-)"
+else
+  echo >&2 "ERROR: cannot find @bazel_tools//tools/bash/runfiles:runfiles.bash"
+  exit 1
+fi
+# --- end runfiles.bash initialization ---
+
+[[ "$$1" = external/* ]] && F1="$${1#external/}" || F1="$$TEST_WORKSPACE/$$1"
+[[ "$$2" = external/* ]] && F2="$${2#external/}" || F2="$$TEST_WORKSPACE/$$2"
+F1="$$(rlocation "$$F1")"
+F2="$$(rlocation "$$F2")"
+diff -q "$$F1" "$$F2"
+eof
+""",
     )
     native.sh_test(
         name = src + "_checkshtest",
         size = "small",
         srcs = [src + "_check.sh"],
+        deps = ["@bazel_tools//tools/bash/runfiles"],
         data = [src, gen],
         args = ["$(location " + src + ")", "$(location " + gen + ")"],
     )