merge(third_party/git): Merge squashed git subtree at v2.23.0

Merge commit '1b593e1ea4' as 'third_party/git'

third_party/git/Documentation/RelNotes/2.0.5.txt

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+Git v2.0.5 Release Notes
+========================
+
+Fixes since v2.0.4
+------------------
+
+ * We used to allow committing a path ".Git/config" with Git that is
+   running on a case sensitive filesystem, but an attempt to check out
+   such a path with Git that runs on a case insensitive filesystem
+   would have clobbered ".git/config", which is definitely not what
+   the user would have expected.  Git now prevents you from tracking
+   a path with ".Git" (in any case combination) as a path component.
+
+ * On Windows, certain path components that are different from ".git"
+   are mapped to ".git", e.g. "git~1/config" is treated as if it were
+   ".git/config".  HFS+ has a similar issue, where certain unicode
+   codepoints are ignored, e.g. ".g\u200cit/config" is treated as if
+   it were ".git/config".  Pathnames with these potential issues are
+   rejected on the affected systems.  Git on systems that are not
+   affected by this issue (e.g. Linux) can also be configured to
+   reject them to ensure cross platform interoperability of the hosted
+   projects.
+
+ * "git fsck" notices a tree object that records such a path that can
+   be confused with ".git", and with receive.fsckObjects configuration
+   set to true, an attempt to "git push" such a tree object will be
+   rejected.  Such a path may not be a problem on a well behaving
+   filesystem but in order to protect those on HFS+ and on case
+   insensitive filesystems, this check is enabled on all platforms.
+
+A big "thanks!" for bringing this issue to us goes to our friends in
+the Mercurial land, namely, Matt Mackall and Augie Fackler.
+
+Also contains typofixes, documentation updates and trivial code clean-ups.