Allow push only per remote branch name

*Remote* branch name must be listed along with 'PUSH' keyword.

This requires predictable name of the remote, but as this whole slurping
thing is only meant for very dirty, very local scenarios, that should
not be problem.  (Other, more open scenarios are still possible, just
harder, as they probably should be.)

dotfiles/gittum/bin/slurp

 
 allowed_push() {
     #
-    # Is push allowed in this repo?
+    # Is push allowed for the remote branch??
     #
-    grep -qxF PUSH "$ok_file" 2>/dev/null
+    # If this branch is tracking a remote branch, and the remote
+    # branch name is listed in .git-slurp-ok after PUSH, we are
+    # allowed to push
+    #
+    # The remote tracking branch is parsed; example output is here:
+    #
+    #     $ git branch -vv
+    #       main   aaf02f0 [main/master: ahead 25] Some other commit
+    #     * master add0a03 [jdsumsion/master] Some commit
+    #
+    local tbranch=$(
+        git branch -vv \
+          | grep '^\*' \
+          | cut -d \] -f1 \
+          | cut -d \[ -f2 \
+          | cut -d :  -f1
+    )
+    test -n "$tbranch" || return 0
+    grep -qxF "PUSH $tbranch" "$ok_file" 2>/dev/null
 }
 
 git_relative() {