.gitlab-ci.d/buildtest.yml: Work around htags bug when environment is large

Sometimes the CI "pages" job fails with a message like this from
htags:

$ htags -anT --tree-view=filetree -m qemu_init -t "Welcome to the QEMU sourcecode"
htags: Negative exec line limit = -371

This is due to a bug in hflags where if the environment is too large it
falls over:
https://lists.gnu.org/archive/html/bug-global/2024-01/msg00000.html

This happens to us because GitLab CI puts the commit message of the
commit under test into the CI_COMMIT_MESSAGE and/or CI_COMMIT_TAG_MESSAGE
environment variables, so the job will fail if the commit happens to
have a verbose commit message.

Work around the htags bug by unsetting these variables while running
htags.

Cc: qemu-stable@nongnu.org
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2080
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20240111125543.1573473-1-peter.maydell@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>
(cherry picked from commit 52a21689cd)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

.gitlab-ci.d/buildtest.yml

@@ -584,7 +584,10 @@ pages:
     - mkdir -p public
     # HTML-ised source tree
     - make gtags
-    - htags -anT --tree-view=filetree -m qemu_init
+    # We unset variables to work around a bug in some htags versions
+    # which causes it to fail when the environment is large
+    - CI_COMMIT_MESSAGE= CI_COMMIT_TAG_MESSAGE= htags
+        -anT --tree-view=filetree -m qemu_init
         -t "Welcome to the QEMU sourcecode"
     - mv HTML public/src
     # Project documentation