Rename determineMergeBaseCommitOid()

The name suggests that the function computes the merge base, which for
Git means specifically the best common ancestors between multiple
commits or branches (see `git merge-base`).

But what the function actually does is to calculate the HEAD commit of
the PR base branch, as derived from the PR merge commit that the action
analyzes. So even though the function has to do with "merge" and "base",
using the term "merge base" is still misleading at best.

This commit renames the function to determineBaseBranchHeadCommitOid(),
which more clearly indicates what the function does.

src/upload-lib.ts

@@ -609,7 +609,7 @@ export async function uploadFiles(
     checkoutURI,
     environment,
     toolNames,
-    await actionsUtil.determineMergeBaseCommitOid(),
+    await actionsUtil.determineBaseBranchHeadCommitOid(),
   );
 
   // Log some useful debug info about the info