cloudstorage: some cleanup of cloud url handling

We know the preference is never empty, so stop testing for this. But
don't maintain two different preferences with basically the same
content. Instead add the '/git' suffix where needed and keep this all in
one place.

Simplify the extraction of the branch name from the cloud URL.

Also a typo fix and a new comment.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

core/checkcloudconnection.cpp

@@ -19,6 +19,7 @@ CheckCloudConnection::CheckCloudConnection(QObject *parent) :
 
 }
 
+// our own madeup API to make sure we are talking to a Subsurface cloud server
 #define TEAPOT "/make-latte?number-of-shots=3"
 #define HTTP_I_AM_A_TEAPOT 418
 #define MILK "Linus does not like non-fat milk"