[PATCH] Enable and fix support for base less merges.

Let the merge strategies handle the base less case if they are able to
do it. It also fixes git-resolve.sh to die if no common ancestors
exists, instead of doing the wrong thing. Furthermore, it contains a
small independent fix for git-merge.sh and a fix for a base less code
path in gitMergeCommon.py.

With this it's possible to use
    git merge -s recursive 'merge message' A B
to do a base less merge of A and B.

[jc: Thanks Fredrik for fixing the brown-paper-bag in git-merge.
 I fixed a small typo in git-merge-resolve fix; 'test' equality
 check is spelled with single equal sign -- C-style double equal
 sign is bashism.]

Signed-off-by: Fredrik Kuivinen <freku045@student.liu.se>
Signed-off-by: Junio C Hamano <junkio@cox.net>
diff --git a/gitMergeCommon.py b/gitMergeCommon.py
index 7e8855c..1b5bddd 100644
--- a/gitMergeCommon.py
+++ b/gitMergeCommon.py
@@ -213,7 +213,7 @@
 
 # Write the empty tree to the object database and return its SHA1
 def writeEmptyTree():
-    tmpIndex = os.environ['GIT_DIR'] + '/merge-tmp-index'
+    tmpIndex = os.environ.get('GIT_DIR', '.git') + '/merge-tmp-index'
     def delTmpIndex():
         try:
             os.unlink(tmpIndex)