Fix the way PR build clones gVisor.

Copybara force-pushes to the PR immediately before merging which triggers a PR
build. Since the PR is merged, the refspec +refs/pull/{pr_num}/merge is not
available and the build fails causing all master commit CI builds to show a
failure.

This change remove the clone step from travis and clone manually in a way
which always would succeed. We fetch +refs/pull/{pr_num}/head and cherry pick
that onto the target branch.

I have tested this in https://github.com/ayushr2/gvisor/pull/1 and
https://github.com/ayushr2/gvisor/pull/2.

PiperOrigin-RevId: 317759891

.travis.yml

@@ -1,5 +1,27 @@
 language: shell
 dist: xenial
+git:
+  clone: false  # Clone manually in before_install
+before_install:
+    - set -e -o pipefail
+    - |
+      if [ "${TRAVIS_PULL_REQUEST}" = false ]; then
+        # This is not a PR build, fetch and checkout the commit being tested
+        git clone -q --depth 1 "https://github.com/${TRAVIS_REPO_SLUG}.git" "${TRAVIS_REPO_SLUG}"
+        cd "${TRAVIS_REPO_SLUG}"
+        git fetch origin "${TRAVIS_COMMIT}" --depth 1
+        git checkout -qf "${TRAVIS_COMMIT}"
+      else
+        # This is a PR build, simulate +refs/pull/{num}/merge.
+        # We can do that by fetching +refs/pull/{num}/head and cherry picking it
+        # onto the target branch.
+        git clone -q --branch "${TRAVIS_BRANCH}" --depth 1 "https://github.com/${TRAVIS_REPO_SLUG}.git" "${TRAVIS_REPO_SLUG}"
+        cd "${TRAVIS_REPO_SLUG}"
+        git fetch origin "+refs/pull/${TRAVIS_PULL_REQUEST}/head" --depth 1
+        git config --global user.email "$(git log -1 FETCH_HEAD --pretty="%cE")"
+        git config --global user.name "$(git log -1 FETCH_HEAD --pretty="%aN")"
+        git cherry-pick --strategy=recursive -X theirs --keep-redundant-commits FETCH_HEAD
+      fi
 cache:
   directories:
     - /home/travis/.cache/bazel/