merge-recursive: Fix support for branch names containing slashes

A branch name could have a slash in it.

Signed-off-by: Fredrik Kuivinen <freku045@student.liu.se>
Signed-off-by: Junio C Hamano <junkio@cox.net>

git-merge-recursive.py

@@ -295,6 +295,7 @@ def uniquePath(path, branch):
             else:
                 raise
 
+    branch = branch.replace('/', '_')
     newPath = path + '_' + branch
     suffix = 0
     while newPath in currentFileSet or \