Fix ABI check on GitHub Actions

* The default is fetching a single commit which contains the whole history,
  which causes the check to believe ABI_version.txt / ABI_check.txt were
  updated when they were actually not changed in the PR.
* See #2871 (comment)

Author: eregon

.github/workflows/ci.yml

@@ -12,6 +12,8 @@ jobs:
     steps:
     - name: Clone TruffleRuby
       uses: actions/checkout@v2
+      with:
+        fetch-depth: 0 # Necessary for jt check_abi
     - name: Setup system Ruby
       uses: ruby/setup-ruby@v1
     - name: Setup jt

tool/jt.rb

@@ -3053,7 +3053,7 @@ def lint(*args)
   ABI_CHECK_FILE = 'lib/cext/ABI_check.txt'
 
   def check_abi(fail: true)
-    puts bold 'ABI check'
+    puts bold '$ jt check_abi'
     # Check since the last commit at which ABI_CHECK_FILE or ABI_VERSION_FILE were modified
     base_commit = `git log -n 1 --format=%H #{ABI_VERSION_FILE} #{ABI_CHECK_FILE}`.chomp