Use git to install MediaWiki (#4)

Rather than downloading and extracting the release file, use git (via the
`actions/checkout@v2` GitHub action) to install MediaWiki. By default this will
not include the history of the repo (no change from prior download method) but
will include files that MediaWiki specifies should not be exported (change from
prior download method that motivates this).

SEL-558

action.yml

@@ -34,8 +34,15 @@ runs:
         path: ~/.composer/cache
         key: composer-php${{ inputs.php }}
 
+    - name: Download Mediawiki
+      uses: actions/checkout@v2
+      with:
+        path: mediawiki
+        repository: wikimedia/mediawiki
+        ref: ${{ inputs.mwbranch }}
+
     - name: Install MediaWiki
-      run: bash $GITHUB_ACTION_PATH/install.sh ${{ inputs.mwbranch }} ${{ inputs.extension }} ${{ inputs.type }}
+      run: bash $GITHUB_ACTION_PATH/install.sh ${{ inputs.extension }} ${{ inputs.type }}
       shell: bash
 
     - uses: actions/checkout@v2

install.sh

@@ -2,16 +2,8 @@
 
 set -o pipefail
 
-MW_BRANCH=$1
-EXTENSION_NAME=$2
-TYPE=$3
-
-# Download wiki release
-wget https://github.com/wikimedia/mediawiki/archive/$MW_BRANCH.tar.gz -nv -q
-
-# Extract into `mediawiki` directory
-tar -zxf $MW_BRANCH.tar.gz
-mv mediawiki-$MW_BRANCH mediawiki
+EXTENSION_NAME=$1
+TYPE=$2
 
 # Install composer dependencies
 cd mediawiki && composer install