Fix webhook commits wrong hash on HEAD reset (#16283)

Use `..` instead of `...` with `rev-list`. In combination with #16282 the receiver can get the correct commit. The behaviour is now like Github.

fixes #11802

modules/git/tests/repos/repo4_commitsbetween/config

@@ -0,0 +1,7 @@
+[core]
+	repositoryformatversion = 0
+	filemode = false
+	bare = false
+	logallrefupdates = true
+	symlinks = false
+	ignorecase = true