Re-apply GIT_COMMIT_ID check for checkpatch

As it turns out, Gerrit's merge commits don't always respect that format
so these mistakes have to be ignored as false positives.

Change-Id: I4e38d9c34c95588e7916fba4c154f017d8c92dec
Signed-off-by: John Tsichritzis <john.tsichritzis@arm.com>

.checkpatch.conf

@@ -43,6 +43,10 @@
 # Commit messages might contain a Gerrit Change-Id.
 --ignore GERRIT_CHANGE_ID
 
+# Do not check the format of commit messages, as Gerrit's merge commits do not
+# preserve it.
+--ignore GIT_COMMIT_ID
+
 # FILE_PATH_CHANGES reports this kind of message:
 # "added, moved or deleted file(s), does MAINTAINERS need updating?"
 # We do not use this MAINTAINERS file process in TF.