Fix behaviour if you have a branch named the same as your --prefix

We were trying to 'git checkout $prefix', which is ambiguous if $prefix
names a directory *and* a branch.  Do 'git checkout -- $prefix' instead.

The main place this appeared was in 'git subtree add'.

Reported by several people.

git-subtree.sh

@@ -426,7 +426,7 @@ cmd_add()
 
 	debug "Adding $dir as '$rev'..."
 	git read-tree --prefix="$dir" $rev || exit $?
-	git checkout "$dir" || exit $?
+	git checkout -- "$dir" || exit $?
 	tree=$(git write-tree) || exit $?
 
 	headrev=$(git rev-parse HEAD) || exit $?

test.sh

@@ -78,6 +78,7 @@ git init
 create main4
 git commit -m 'main4'
 git branch -m master mainline
+git branch subdir
 
 git fetch ../subproj sub1
 git branch sub1 FETCH_HEAD

todo

@@ -20,9 +20,6 @@
 
 	automated tests for --squash stuff
 
-	test.sh fails in msysgit?
-		sort error - see Thell's email
-
 	"add" command non-obviously requires a commitid; would be easier if
 		it had a "pull" sort of mode instead
 
@@ -43,9 +40,6 @@
 		should detect (and fix) it if it does.  Otherwise the
 		log message looks weird.
 
-	totally weird behavior in 'git subtree add' if --prefix matches 
-		a branch name
-
 	"pull --squash" should do fetch-synthesize-merge, but instead just
 		does "pull" directly, which doesn't work at all.