Replace AppVeyor for GitHub Actions workflow for Windows builds

Note AppVeyor will need turning off in settings too or the tests will
start and fail due to the removal of the config file.

.ci_helpers/clone

@@ -18,7 +18,7 @@ ref=''
 [ -z "$ref" ] && repository='https://github.com/samtools/htslib.git'
 
 set -x
-git clone --recurse-submodules --shallow-submodules --depth=1 ${ref:+--branch="$branch"} "$repository" "$localdir"
+git clone --recurse-submodules --shallow-submodules --depth=2 ${ref:+--branch="$branch"} "$repository" "$localdir"
 
 # NB: "samtools" as the owner/organisation, not the repo name
 if [ "x$owner" = "xsamtools" -a -z "$ref" -a "x$htslib_PR" != "x" ]

.gitattributes

@@ -10,3 +10,6 @@
 .git*           export-ignore
 .ci_helpers     export-ignore
 README.md       export-ignore
+
+# Prevent Windows cr-lf endings.
+test/**        -text

.github/workflows/windows-build.yml

@@ -0,0 +1,52 @@
+name: Windows/MinGW-W64 CI
+on: [push, pull_request]
+
+jobs:
+  build:
+    runs-on: windows-latest
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v4
+      with:
+        ref: ${{ github.event.pull_request.head.sha }}
+    - name: Set up MSYS2 MinGW-W64
+      uses: msys2/setup-msys2@v2
+      with:
+        msystem: mingw64
+        update: false
+        install: >-
+          mingw-w64-x86_64-toolchain
+          mingw-w64-x86_64-autotools
+          mingw-w64-x86_64-curl
+          mingw-w64-x86_64-libdeflate
+          mingw-w64-x86_64-tools-git
+          mingw-w64-x86_64-zlib
+          mingw-w64-x86_64-bzip2
+          mingw-w64-x86_64-xz
+    - name: Clone htslib
+      shell: msys2 {0}
+      run: |
+        export PATH="$PATH:/mingw64/bin:/c/Program Files/Git/bin"
+        export MSYSTEM=MINGW64
+        htslib_pr=`git log -2 --format='%s' | sed -n 's/.*htslib#\([0-9]*\).*/\1/p'`
+        .ci_helpers/clone ${GITHUB_REPOSITORY_OWNER} htslib htslib ${GITHUB_HEAD_REF:-$GITHUB_REF_NAME} $htslib_pr
+        pushd .
+        cd htslib
+        autoreconf -i
+        popd
+    - name: Compile bcftools
+      shell: msys2 {0}
+      run: |
+        export PATH="$PATH:/mingw64/bin:/c/Program Files/Git/bin"
+        export MSYSTEM=MINGW64
+        autoheader
+        autoconf -Wno-syntax
+        ./configure --enable-werror
+        make -j4
+    - name: Check bcftools
+      shell: msys2 {0}
+      run: |
+        export PATH="$PATH:/mingw64/bin:/c/Program Files/Git/bin"
+        export MSYSTEM=MINGW64
+        make check
+