Add check-owners-pr.sh wrapper around check-owners

```
$ dev/tools/check-owners-pr.sh 6809 --show-patterns --owner '@MSoegtropIMC'
remote: Counting objects: 93, done.
remote: Compressing objects: 100% (3/3), done.
remote: Total 93 (delta 47), reused 50 (delta 47), pack-reused 43
Unpacking objects: 100% (93/93), done.
From github.com:coq/coq
 * branch                  refs/pull/6809/head -> FETCH_HEAD
 * branch                  master     -> FETCH_HEAD
/dev/build/windows: @MSoegtropIMC
```

1 files changed, 32 insertions, 0 deletions

diff --git a/dev/tools/check-owners-pr.sh b/dev/tools/check-owners-pr.sh
new file mode 100755
index 000000000..d2910279b
--- /dev/null
+++ b/dev/tools/check-owners-pr.sh
@@ -0,0 +1,32 @@
+#!/usr/bin/env sh
+
+usage() {
+    { echo "usage: $0 PR [ARGS]..."
+      echo "A wrapper around check-owners.sh to check owners for a PR."
+      echo "Assumes upstream is the canonical Coq repository."
+      echo "Assumes the PR is against master."
+      echo
+      echo "  PR: PR number"
+      echo "  ARGS: passed through to check-owners.sh"
+    } >&2
+}
+
+case "$1" in
+    "--help"|"-h")
+        usage
+        if [ $# = 1 ]; then exit 0; else exit 1; fi;;
+    "")
+        usage
+        exit 1;;
+esac
+
+PR="$1"
+shift
+
+# this puts both refs in the FETCH_HEAD file but git rev-parse will use the first
+git fetch upstream "+refs/pull/$PR/head" master
+
+head=$(git rev-parse FETCH_HEAD)
+base=$(git merge-base upstream/master "$head")
+
+git diff --name-only -z "$base" "$head" | xargs -0 dev/tools/check-owners.sh "$@"