gitlab-ci: introduce stages and dependencies

We're about to add a couple of jobs for Windows. As the Windows runners
are quite slow, we will split those up across two stages: one stage to
build the artifacts, and one stage that runs test slices in parallel.

Introduce stages and "needs" dependencies for the preexisting jobs as a
preparatory step. The stages will lead to a more natural representation
of jobs in the UI, whereas the "needs" dependency ensures that jobs do
not have to wait for all jobs in the preceding stage to finish.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

.gitlab-ci.yml

@@ -1,6 +1,10 @@
 default:
   timeout: 2h
 
+stages:
+  - test
+  - analyze
+
 workflow:
   rules:
     - if: $CI_PIPELINE_SOURCE == "merge_request_event"
@@ -9,6 +13,8 @@ workflow:
 
 test:linux:
   image: $image
+  stage: test
+  needs: [ ]
   tags:
     - saas-linux-medium-amd64
   variables:
@@ -67,6 +73,8 @@ test:linux:
 
 test:osx:
   image: $image
+  stage: test
+  needs: [ ]
   tags:
     - saas-macos-medium-m1
   variables:
@@ -102,6 +110,8 @@ test:osx:
 
 test:fuzz-smoke-tests:
   image: ubuntu:latest
+  stage: test
+  needs: [ ]
   variables:
     CC: clang
   before_script:
@@ -111,6 +121,8 @@ test:fuzz-smoke-tests:
 
 static-analysis:
   image: ubuntu:22.04
+  stage: analyze
+  needs: [ ]
   variables:
     jobname: StaticAnalysis
   before_script:
@@ -121,6 +133,8 @@ static-analysis:
 
 check-whitespace:
   image: ubuntu:latest
+  stage: analyze
+  needs: [ ]
   before_script:
     - ./ci/install-dependencies.sh
   # Since $CI_MERGE_REQUEST_TARGET_BRANCH_SHA is only defined for merged
@@ -135,6 +149,8 @@ check-whitespace:
 
 check-style:
   image: ubuntu:latest
+  stage: analyze
+  needs: [ ]
   allow_failure: true
   variables:
     CC: clang
@@ -153,6 +169,8 @@ check-style:
 
 documentation:
   image: ubuntu:latest
+  stage: analyze
+  needs: [ ]
   variables:
     jobname: Documentation
   before_script: