GitLab CI: Speed up our fetching/cloning.

Fetching the complete Wireshark repository in GitLab's CI currently
takes about 3.5 minutes. GitLab CI lets you do shallow cloning, but only
up to 1000 commits. This isn't sufficient for any of our jobs that might
need a reachable tag since it's currently about 1200 commits away in
master.

Set GIT_DEPTH to 1 (which appears to trigger the required shallow clone
logic in GitLab's CI), then deepen it by setting GIT_FETCH_EXTRA_FLAGS
to "--depth=5000". This is *should* be much faster than a full checkout
and still give us a reachable tag:

$ git rev-list --count v3.3.0rc0..v3.5.0rc0
2330
$ git rev-list --count v3.1.0rc0..v3.3.0rc0
2198
$ git rev-list --count v2.9.0rc0..v3.1.0rc0
3449

.gitlab-ci.yml

@@ -11,6 +11,12 @@
 # https://hub.docker.com/r/wireshark/wireshark-ubuntu-dev
 
 variables:
+  # Ensure that checkouts are a) fast and b) have a reachable tag. In a
+  # brighter, more glorious future we might be able to use --shallow-since:
+  # https://gitlab.com/gitlab-org/gitlab-runner/-/issues/3460
+  # In the mean time, fetching the last 5000 commits does the job.
+  GIT_DEPTH: "1"
+  GIT_FETCH_EXTRA_FLAGS: "--depth=5000"
   CCACHE_DIR: "${CI_PROJECT_DIR}/ccache"
 
 .build:
@@ -264,8 +270,6 @@ merge-req:commit-checks:
     - docker
   rules:
     - if: '$CI_PIPELINE_SOURCE == "merge_request_event"'
-  variables:
-    GIT_DEPTH: 10
   script:
     # build-ubuntu puts us in `build`.
     - cd ..
@@ -278,8 +282,6 @@ merge-req:ubuntu-dpkg:
     - docker
   rules:
     - if: '$CI_PIPELINE_SOURCE == "merge_request_event"'
-  variables:
-    GIT_DEPTH: 10
   script:
     # build-ubuntu puts us in `build`.
     - cd ..
@@ -296,8 +298,6 @@ merge-req:ubuntu-gcc-ctest:
     - docker
   rules:
     - if: '$CI_PIPELINE_SOURCE == "merge_request_event"'
-  variables:
-    GIT_DEPTH: 10
   script:
     # build-ubuntu puts us in `build`.
     - CC=gcc CXX=g++ cmake -DENABLE_EXTRA_COMPILER_WARNINGS=on -DCMAKE_EXPORT_COMPILE_COMMANDS=on -DENABLE_CCACHE=ON -G Ninja ..
@@ -312,8 +312,6 @@ merge-req:ubuntu-clang-other-tests:
     - docker
   rules:
     - if: '$CI_PIPELINE_SOURCE == "merge_request_event"'
-  variables:
-    GIT_DEPTH: 10
   script:
     # build-ubuntu puts us in `build`.
     - cd ..
@@ -348,8 +346,6 @@ merge-req:windows:
     # The wireshark-windows-dev image is only available via a dedicated runner.
     - if: '$CI_PIPELINE_SOURCE == "merge_request_event" && $CI_PROJECT_URL =~ /.*gitlab.com\/wireshark\/wireshark/'
       when: always
-  variables:
-    GIT_DEPTH: 10
   before_script:
     # XXX Find a better location.
     - cmd.exe /c "set CI_PIPELINE_SOURCE"