[travis] Add requirements file for travis and fix diff command (#2406)

.travis.yml

@@ -8,6 +8,6 @@ before_install:
   - git fetch origin +refs/heads/$TRAVIS_BRANCH
 
 install:
-  - pip install -r infra/gcb/requirements.txt
+  - pip install -r infra/travis/requirements.txt
 
 script: ./infra/travis/travis_build.py

infra/travis/requirements.txt

@@ -0,0 +1 @@
+PyYAML==5.1

infra/travis/travis_build.py

@@ -30,8 +30,10 @@ DEFAULT_SANITIZERS = ['address', 'undefined']
 
 def get_modified_projects():
   """Get a list of all the projects modified in this commit."""
-  output = subprocess.check_output(['git', 'diff', 'origin/master',
-                                    '--name-status'])
+  master_head_sha = subprocess.check_output(
+      ['git', 'merge-base', 'HEAD', 'FETCH_HEAD']).decode().strip()
+  output = subprocess.check_output(
+      ['git', 'diff', '--name-only', 'HEAD', master_head_sha]).decode()
   projects_regex = '.*projects/(?P<name>.*)/.*\n'
   return set(re.findall(projects_regex, output))