Correct snowball download/unzip sequence to be always consistent.

gradle/generation/snowball.gradle

@@ -38,19 +38,27 @@ configure(project(":lucene:analysis:common")) {
     snowballScript     = rootProject.file("gradle/generation/snowball.sh")
   }
 
+  def unpackFromZip = { zipFile, targetDir ->
+    project.sync {
+      from(zipTree(zipFile), {
+        eachFile { fcd ->
+          fcd.relativePath = new RelativePath(true, fcd.relativePath.segments.drop(1))
+        }
+      })
+      into targetDir
+    }
+  }
+
   // downloads snowball stemmers (or use cached copy)
   task downloadSnowballStemmers(type: Download) {
     inputs.file(snowballPatchFile)
     src "https://github.com/snowballstem/snowball/archive/${snowballStemmerCommit}.zip"
-    def snowballStemmerZip = file("${snowballStemmerDir}.zip")
-    dest snowballStemmerZip
+    dest file("${snowballStemmerDir}.zip")
     overwrite false
     tempAndMove true
 
     doLast {
-      ant.unzip(src: snowballStemmerZip, dest: snowballStemmerDir, overwrite: "true") {
-        ant.cutdirsmapper(dirs: "1")
-      }
+      unpackFromZip(dest, snowballStemmerDir)
       ant.patch(patchfile: snowballPatchFile, dir: snowballStemmerDir, strip: "1", failonerror: true)
     }
   }
@@ -64,9 +72,7 @@ configure(project(":lucene:analysis:common")) {
     tempAndMove true
 
     doLast {
-      ant.unzip(src: snowballWebsiteZip, dest: snowballWebsiteDir, overwrite: "true") {
-        ant.cutdirsmapper(dirs: "1")
-      }
+      unpackFromZip(snowballWebsiteZip, snowballWebsiteDir)
     }
   }
 
@@ -79,9 +85,7 @@ configure(project(":lucene:analysis:common")) {
     tempAndMove true
 
     doLast {
-      ant.unzip(src: snowballDataZip, dest: snowballDataDir, overwrite: "true") {
-        ant.cutdirsmapper(dirs: "1")
-      }
+      unpackFromZip(snowballDataZip, snowballDataDir)
     }
   }
 
@@ -90,11 +94,11 @@ configure(project(":lucene:analysis:common")) {
     description "Regenerates snowball stemmers."
     group "generation"
 
-    // Don't even bother adding dependencies.
+    // Don't even bother adding dependencies on Windows.
     if (Os.isFamily(Os.FAMILY_WINDOWS)) {
       doFirst {
         // Just emit a big fat error message but don't fail the build.
-        logger.error("Snowball generation does not work on Windows, use a platform where bash is available.")
+        logger.error("Snowball generation does not work on Windows (patch and bash must be available).")
       }
     } else {
       dependsOn downloadSnowballStemmers