Ignore user home during tests

As of JGit 4.9.0, they support reading the pull.ff git-config variable.
I had this configured on my system, which caused some tests to fail due
to expectations that isn't set. Setting the user.home to null avoids
this from happening in the first place.

src/test/groovy/org/ajoberstar/grgit/fixtures/MultiGitOpSpec.groovy

@@ -33,6 +33,10 @@ class MultiGitOpSpec extends Specification {
   protected Grgit init(String name) {
     File repoDir = tempDir.newFolder(name).canonicalFile
     Git git = Git.init().setDirectory(repoDir).call()
+
+    // Don't want the user's git config to conflict with test expectations
+    git.repo.FS.userHome = null
+
     git.repo.config.with {
       setString('user', null, 'name', person.name)
       setString('user', null, 'email', person.email)

src/test/groovy/org/ajoberstar/grgit/fixtures/SimpleGitOpSpec.groovy

@@ -34,6 +34,10 @@ class SimpleGitOpSpec extends Specification {
   def setup() {
     File repoDir = tempDir.newFolder('repo')
     Git git = Git.init().setDirectory(repoDir).call()
+
+    // Don't want the user's git config to conflict with test expectations
+    git.repo.FS.userHome = null
+
     git.repo.config.with {
       setString('user', null, 'name', person.name)
       setString('user', null, 'email', person.email)