[GR-18163] Only take the last commit to find the base commit for checking ABI

PullRequest: truffleruby/2629

Author: eregon

tool/jt.rb

@@ -2710,7 +2710,7 @@ def lint(*args)
 
   def check_abi
     # Check since the last commit at which ABI_CHECK_FILE or ABI_VERSION_FILE were modified
-    base_commit = `git log --format=%H #{ABI_VERSION_FILE} #{ABI_CHECK_FILE}`.chomp
+    base_commit = `git log -n 1 --format=%H #{ABI_VERSION_FILE} #{ABI_CHECK_FILE}`.chomp
 
     changed_files = changed_files(base_commit)
     # All files which can affect the ABI of libtruffleruby.so