Sync with 2.3.10

Documentation/RelNotes/2.3.10.txt

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+Git v2.3.10 Release Notes
+=========================
+
+Fixes since v2.3.9
+------------------
+
+ * xdiff code we use to generate diffs is not prepared to handle
+   extremely large files.  It uses "int" in many places, which can
+   overflow if we have a very large number of lines or even bytes in
+   our input files, for example.  Cap the input size to soemwhere
+   around 1GB for now.
+
+ * Some protocols (like git-remote-ext) can execute arbitrary code
+   found in the URL.  The URLs that submodules use may come from
+   arbitrary sources (e.g., .gitmodules files in a remote
+   repository), and can hurt those who blindly enable recursive
+   fetch.  Restrict the allowed protocols to well known and safe
+   ones.