Merge #11699: [travis-ci] Only run linters on Pull Requests

ec85248 [travis-ci] Only run linters on Pull Requests (John Newbery)

Pull request description:

  Linters should only be run on pull request builds. Once bad code style
  has been merged into master, it's too late. Master and other branches
  should not fail to build because of linter warnings.

Tree-SHA512: f8e56e0c338db60110a67bdd39a5837b42e156180349a81a68a383a5adef07ecf7b4946c1565333b47edd2b1a70e52caf0600bdf448f6eb01fa47595e8df82c9

.travis.yml

@@ -52,7 +52,7 @@ before_script:
     - if [ "$CHECK_DOC" = 1 ]; then contrib/devtools/git-subtree-check.sh src/leveldb; fi
     - if [ "$CHECK_DOC" = 1 ]; then contrib/devtools/check-doc.py; fi
     - if [ "$CHECK_DOC" = 1 ]; then contrib/devtools/check-rpc-mappings.py .; fi
-    - if [ "$CHECK_DOC" = 1 ]; then contrib/devtools/lint-all.sh; fi
+    - if [ "$CHECK_DOC" = 1 -a "$TRAVIS_EVENT_TYPE" = "pull_request" ]; then contrib/devtools/lint-all.sh; fi
     - unset CC; unset CXX
     - mkdir -p depends/SDKs depends/sdk-sources
     - if [ -n "$OSX_SDK" -a ! -f depends/sdk-sources/MacOSX${OSX_SDK}.sdk.tar.gz ]; then curl --location --fail $SDK_URL/MacOSX${OSX_SDK}.sdk.tar.gz -o depends/sdk-sources/MacOSX${OSX_SDK}.sdk.tar.gz; fi