Fix GHCommitPointer null repository path and add regression test

[n09tal]

Fix issue #2035: handle null repository in GHCommitPointer#getCommit and add regression test.

What changed

1. src/main/java/org/kohsuke/github/GHCommitPointer.java

• Updated getCommit() to avoid an uncontrolled NullPointerException path.
• Added a local repository variable from getRepository().
• Added a null guard:
  • If repository is null, throw IOException("Cannot commit because repository is null.").
  • Otherwise continue with repository.getCommit(getSha()).

2. src/test/java/org/kohsuke/github/GHCommitPointerTest.java (new file)

• Added regression test nullRepoNpe.
• Test creates a GHCommitPointer with default null repository state.
• Verifies getCommit() now throws IOException (not NPE).
• Verifies exception message contains "repository is null".

3. src/main/java/org/kohsuke/github/GHPullRequest.java

• Minor inline comment update in getHead() for debugging context.
• No functional logic change.

Why

Issue #2035 reports an NPE when GHCommitPointer#getCommit() is called and repository data is missing.
This change makes behavior explicit and controlled, and adds test coverage for the null-repository path.

Validation

• Targeted test command:
  • mvn -Dtest=GHCommitPointerTest#nullRepoNpe test
• Result:
  • Tests run: 1, Failures: 0, Errors: 0
  • BUILD SUCCESS