git/branch: don't clobber variable in merge1 argument

When doing a merge on branch switch, we would use 'base'
as the base commmit and later override it with the merge
ancestor of the commit, fucking up the paths.

Rename the variable so we don't clobber it.

branch

@@ -95,9 +95,9 @@ for(m in $cleanpaths){
 }
 
 for(ours in $dirtypaths){
-	base=/mnt/git/object/$orig/tree/$ours
+	common=/mnt/git/object/$orig/tree/$ours
 	theirs=/mnt/git/object/$base/tree/$ours
-	merge1 $ours $ours $base $theirs
+	merge1 $ours $ours $common $theirs
 }
 
 if(! ~ $#deleted 0){