CI: Break out base commit logic from get_changed_files() (MR 6301)

This makes the code more self-documenting and allows re-using this logic
in other parts of the code.

.ci/lib/common.py

@@ -86,12 +86,10 @@ def get_upstream_branch():
     return ret
 
 
-def get_changed_files(removed=True):
-    """ Get all changed files and print them, as well as the branch and the
-        commit that was used for the diff.
-        :param removed: also return removed files (default: True)
-        :returns: set of changed files
-    """
+def get_base_commit() -> str:
+    """Get the base commit that can be compared with HEAD to get all commits in the
+    given branch/merge request."""
+
     branch_upstream = f"upstream/{get_upstream_branch()}"
     commit_head = run_git(["rev-parse", "HEAD"])[:-1]
     commit_upstream = run_git(["rev-parse", branch_upstream])[:-1]
@@ -107,6 +105,17 @@ def get_changed_files(removed=True):
         commit = run_git(["merge-base", branch_upstream, "HEAD"])[:-1]
     print("comparing HEAD with: " + commit)
 
+    return commit
+
+
+def get_changed_files(removed=True):
+    """ Get all changed files and print them, as well as the branch and the
+        commit that was used for the diff.
+        :param removed: also return removed files (default: True)
+        :returns: set of changed files
+    """
+    commit = get_base_commit()
+
     # Changed files
     ret = set()
     print("changed file(s):")