fix: make merge base fetching more robust

Several static check scripts rely on identifying the Git merge base via
get_merge_base, which may fail if the local Git history is too shallow.
This causes issues when checking patches on CI or LTS branches with
limited fetch depth.

To address this:

- Reuse a single evaluated merge_base instead of invoking get_merge_base
  multiple times.
- If merge_base is not found initially, attempt to fetch more history
  from the remote using the appropriate GERRIT_REFSPEC.
- Exit with an error if a merge base cannot be found even after
  fetching.

This improves reliability of static checks when working on partial
clones or CI environments where fetch depth is restricted.

Change-Id: Icccec0eb2f29d254e54bbd6b639f6c1ef11291a3
Signed-off-by: Harrison Mutai <harrison.mutai@arm.com>
diff --git a/script/static-checks/static-checks-banned-apis.sh b/script/static-checks/static-checks-banned-apis.sh
index 4bab6a7..424bbcb 100755
--- a/script/static-checks/static-checks-banned-apis.sh
+++ b/script/static-checks/static-checks-banned-apis.sh
@@ -17,7 +17,7 @@
   echo "# Check for banned APIs in the patch"
   TEST_CASE="Banned API check on patch(es)"
   "$CI_ROOT/script/static-checks/check-banned-api.py" --tree "$1" \
-      --patch --from-ref $(get_merge_base) \
+      --patch --from-ref ${merge_base} \
       &> "$LOG_FILE"
 else
   echo "# Check for banned APIs in entire source tree"