Support multiple %base arguments with refs/for/

Multiple %base arguments are necessary when pushing a merge commit
in order to mark the uninteresting part of the TreeWalk for each
parent line.

Example:
  $ git push origin HEAD:refs/for/master%base=commitId1,base=commitId2

Change-Id: Ie291e7eb2d5755d7c69b5625662af03561de1386

Documentation/user-upload.txt

diff --git a/Documentation/user-upload.txt b/Documentation/user-upload.txt
index 0569630..17c4acf 100644
--- a/Documentation/user-upload.txt
+++ b/Documentation/user-upload.txt
@@ -366,6 +366,15 @@
   git push ssh://john.doe@git.example.com:29418/kernel/common HEAD:refs/for/master%base=$(git rev-parse origin/master)
 ====
 
+It is also possible to specify more than one '%base' argument.
+This may be useful when pushing a merge commit. Note that the '%'
+character has only to be provided once, for the first '%base'
+argument:
+
+====
+  git push ssh://john.doe@git.example.com:29418/kernel/common HEAD:refs/for/master%base=commit-id1,base=commit-id2
+====
+
 
 repo upload
 -----------