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/objects/78/a445db1eac62fe15e624e1137965969addf344

@@ -0,0 +1,3 @@
+x
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+<EFBFBD><EFBFBD>Dm<08>ڃ<06><>w<EFBFBD><77><1F>ź{p<>C_