Correct GIT_INDEX_FILE environment variable

This is GIT_INDEX_FILE, not GIT_INDEX.

Change-Id: Ib3af28ba196f74c8cb4d318b57ea346bb90f9a1e
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

org.eclipse.jgit/src/org/eclipse/jgit/lib/BaseRepositoryBuilder.java

 import static org.eclipse.jgit.lib.Constants.GIT_ALTERNATE_OBJECT_DIRECTORIES_KEY;
 import static org.eclipse.jgit.lib.Constants.GIT_CEILING_DIRECTORIES_KEY;
 import static org.eclipse.jgit.lib.Constants.GIT_DIR_KEY;
-import static org.eclipse.jgit.lib.Constants.GIT_INDEX_KEY;
+import static org.eclipse.jgit.lib.Constants.GIT_INDEX_FILE_KEY;
 import static org.eclipse.jgit.lib.Constants.GIT_OBJECT_DIRECTORY_KEY;
 import static org.eclipse.jgit.lib.Constants.GIT_WORK_TREE_KEY;
 
 		}
 
 		if (getIndexFile() == null) {
-			String val = sr.getenv(GIT_INDEX_KEY);
+			String val = sr.getenv(GIT_INDEX_FILE_KEY);
 			if (val != null)
 				setIndexFile(new File(val));
 		}

org.eclipse.jgit/src/org/eclipse/jgit/lib/Constants.java

 	/**
 	 * The environment variable that tells us which file holds the Git index.
 	 */
-	public static final String GIT_INDEX_KEY = "GIT_INDEX";
+	public static final String GIT_INDEX_FILE_KEY = "GIT_INDEX_FILE";
 
 	/**
 	 * The environment variable that tells us where objects are stored