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

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

third_party/git/Documentation/RelNotes/1.5.1.3.txt

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+GIT v1.5.1.3 Release Notes
+==========================
+
+Fixes since v1.5.1.2
+--------------------
+
+* Bugfixes
+
+  - git-add tried to optimize by finding common leading
+    directories across its arguments but botched, causing very
+    confused behaviour.
+
+  - unofficial rpm.spec file shipped with git was letting
+    ETC_GITCONFIG set to /usr/etc/gitconfig.  Tweak the official
+    Makefile to make it harder for distro people to make the
+    same mistake, by setting the variable to /etc/gitconfig if
+    prefix is set to /usr.
+
+  - git-svn inconsistently stripped away username from the URL
+    only when svnsync_props was in use.
+
+  - git-svn got confused when handling symlinks on Mac OS.
+
+  - git-send-email was not quoting recipient names that have
+    period '.' in them.  Also it did not allow overriding
+    envelope sender, which made it impossible to send patches to
+    certain subscriber-only lists.
+
+  - built-in write_tree() routine had a sequence that renamed a
+    file that is still open, which some systems did not like.
+
+  - when memory is very tight, sliding mmap code to read
+    packfiles incorrectly closed the fd that was still being
+    used to read the pack.
+
+  - import-tars contributed front-end for fastimport was passing
+    wrong directory modes without checking.
+
+  - git-fastimport trusted its input too much and allowed to
+    create corrupt tree objects with entries without a name.
+
+  - git-fetch needlessly barfed when too long reflog action
+    description was given by the caller.
+
+Also contains various documentation updates.