fix build scripts when git refs are defined by their branch name

This fixes the case when secp256k1 is already cloned. Other packages are not affected because their git ref is a commit hash.

contrib/base.sh

@@ -176,15 +176,15 @@ function setup_pkg()
     if [ -d "$pkgbuilddir" ] ; then
         pushd "$pkgbuilddir"
         local commit=`git rev-parse HEAD`
-        local branch=`git rev-parse --abbrev-ref`
+        local branch=`git rev-parse --abbrev-ref HEAD`
         local tag=`git describe --tags`
         if [[ ${commit} = ${checkout_ref} || ${branch} = ${checkout_ref} || ${tag} = ${checkout_ref} ]] ; then
             warn "$pkgname already cloned and $checkout_ref is already the current HEAD"
             # Just make sure there are no accidental changes
             git stash
             return
         fi
-        fail "$pkgname already cloned, but HEAD is not at expected ref ${checkout_ref}"
+        fail "$pkgname already cloned, but HEAD is not at expected ref ${checkout_ref} (branch ${branch}, commit ${commit})"
         popd
     fi
     git clone ${git_url}