[BE][GHF] Add retries_decorator (#101227)

I've noticed that 3-4 functions in trymerge are trying to implement similar tail recursion for flaky network retries.

Unify them using single wrapper in `gitutils.py`

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### <samp>🤖 Generated by Copilot at 8d40631</samp>

> _`retries_decorator`_
> _adds resilience to GitHub scripts_
> _autumn of errors_
Pull Request resolved: https://github.com/pytorch/pytorch/pull/101227
Approved by: https://github.com/kit1980

.github/scripts/test_gitutils.py

@@ -8,6 +8,7 @@ from gitutils import (
     GitRepo,
     patterns_to_regex,
     PeekableIterator,
+    retries_decorator,
 )
 
 
@@ -49,6 +50,22 @@ class TestPattern(TestCase):
             self.assertTrue(patterns_re.match(filename))
 
 
+class TestRetriesDecorator(TestCase):
+    def test_simple(self) -> None:
+        @retries_decorator()
+        def foo(x: int, y: int) -> int:
+            return x + y
+
+        self.assertEqual(foo(3, 4), 7)
+
+    def test_fails(self) -> None:
+        @retries_decorator(rc=0)
+        def foo(x: int, y: int) -> int:
+            return x + y
+
+        self.assertEqual(foo("a", 4), 0)
+
+
 class TestGitRepo(TestCase):
     def setUp(self) -> None:
         repo_dir = BASE_DIR.parent.parent.absolute()