Throw GitVersionException instead if not in a Git repository

This allows this error to be recoverable from GradleUtils, since it is
expected for GitVersionExceptions to be caught and potentially
recoverable (i.e. using temporary fake versions such as 0.0.0), while
IllegalArgumentExceptions should be dealt with accordingly.

Author: Jonathing

src/main/java/net/minecraftforge/gitver/internal/GitVersionImpl.java

@@ -53,7 +53,7 @@ public GitVersionImpl(File gitDir, File root, File project, GitVersionConfig con
         this.gitDir = gitDir;
         this.root = root;
         if (!this.gitDir.exists())
-            throw new IllegalArgumentException("Root directory is not a git repository!");
+            throw new GitVersionExceptionInternal("Root directory is not a git repository!");
 
         this.project = project;
         if (this.project.compareTo(this.root) < 0)