ci: handle Windows-based CI jobs in GitLab CI

We try to abstract away any differences between different CI platforms
in "ci/lib.sh", such that knowledge specific to e.g. GitHub Actions or
GitLab CI is neatly encapsulated in a single place. Next to some generic
variables, we also set up some variables that are specific to the actual
platform that the CI operates on, e.g. Linux or macOS.

We do not yet support Windows runners on GitLab CI. Unfortunately, those
systems do not use the same "CI_JOB_IMAGE" environment variable as both
Linux and macOS do. Instead, we can use the "OS" variable, which should
have a value of "Windows_NT" on Windows platforms.

Handle the combination of "$OS,$CI_JOB_IMAGE" and introduce support for
Windows.

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

ci/lib.sh

@@ -250,8 +250,13 @@ then
 	CI_TYPE=gitlab-ci
 	CI_BRANCH="$CI_COMMIT_REF_NAME"
 	CI_COMMIT="$CI_COMMIT_SHA"
-	case "$CI_JOB_IMAGE" in
-	macos-*)
+
+	case "$OS,$CI_JOB_IMAGE" in
+	Windows_NT,*)
+		CI_OS_NAME=windows
+		JOBS=$NUMBER_OF_PROCESSORS
+		;;
+	*,macos-*)
 		# GitLab CI has Python installed via multiple package managers,
 		# most notably via asdf and Homebrew. Ensure that our builds
 		# pick up the Homebrew one by prepending it to our PATH as the
@@ -259,9 +264,12 @@ then
 		export PATH="$(brew --prefix)/bin:$PATH"
 
 		CI_OS_NAME=osx
+		JOBS=$(nproc)
+		;;
+	*,alpine:*|*,fedora:*|*,ubuntu:*)
+		CI_OS_NAME=linux
+		JOBS=$(nproc)
 		;;
-	alpine:*|fedora:*|ubuntu:*)
-		CI_OS_NAME=linux;;
 	*)
 		echo "Could not identify OS image" >&2
 		env >&2
@@ -272,6 +280,7 @@ then
 	CI_JOB_ID="$CI_JOB_ID"
 	CC="${CC_PACKAGE:-${CC:-gcc}}"
 	DONT_SKIP_TAGS=t
+
 	handle_failed_tests () {
 		create_failed_test_artifacts
 		return 1
@@ -280,7 +289,6 @@ then
 	cache_dir="$HOME/none"
 
 	distro=$(echo "$CI_JOB_IMAGE" | tr : -)
-	JOBS=$(nproc)
 else
 	echo "Could not identify CI type" >&2
 	env >&2