ci: split up merge-base failure fetch in static checks

This small change splits up the fetch after a merge-base command failure
in the static checks script into two steps:

1. Unshallow the repository *independently*, then
2. Fetch the desired change.

This is really a last-ditch attempt to resolve issues like:

    ----------------------------------------------
    -- Running static checks on the source code --
    ----------------------------------------------
    /home/buildslave/agent/workspace/tf-a-static-checks/trusted-firmware-a
    error: Could not read 81445dd10705799610307be57125ac4bae1a5730
    fatal: Failed to traverse parents of commit c1ad67a174528f2295fac0f4c728a4d4398aa15c
    error: remote did not send all necessary objects
    fatal: Not a valid object name FETCH_HEAD
    Failed to find merge base, fetching entire change history
    error: Could not read 81445dd10705799610307be57125ac4bae1a5730
    fatal: Failed to traverse parents of commit c1ad67a174528f2295fac0f4c728a4d4398aa15c
    error: remote did not send all necessary objects
    error: pathspec 'FETCH_HEAD' did not match any file(s) known to git
    error: Could not read 81445dd10705799610307be57125ac4bae1a5730
    fatal: Failed to traverse parents of commit c1ad67a174528f2295fac0f4c728a4d4398aa15c
    error: remote did not send all necessary objects
    fatal: Not a valid object name FETCH_HEAD
    Failed to determine merge base after fetching. Exiting.

... because I am very definitely running out of ideas.

Signed-off-by: Chris Kay <chris.kay@arm.com>
Change-Id: Ie3087b117a5096770502042be89cf11d01c6d562
diff --git a/script/static-checks/static-checks.sh b/script/static-checks/static-checks.sh
index 6ff25a8..05747f4 100755
--- a/script/static-checks/static-checks.sh
+++ b/script/static-checks/static-checks.sh
@@ -30,7 +30,8 @@
         fi
     fi
 
-    git fetch --unshallow --update-shallow origin "$GERRIT_REFSPEC"
+    git fetch --unshallow
+    git fetch origin "$GERRIT_REFSPEC"
     git checkout FETCH_HEAD
 
     merge_base=$(get_merge_base)