merge(3p/git): Merge git subtree at v2.29.2

This also bumps the stable nixpkgs to 20.09 as of 2020-11-21, because
there is some breakage in the git build related to the netrc
credentials helper which someone has taken care of in nixpkgs.

The stable channel is not used for anything other than git, so this
should be fine.

Change-Id: I3575a19dab09e1e9556cf8231d717de9890484fb

third_party/git/Documentation/git-svn.txt

@@ -677,7 +677,8 @@ config key: svn.authorsProg
 -s<strategy>::
 --strategy=<strategy>::
 -p::
---preserve-merges::
+--rebase-merges::
+--preserve-merges (DEPRECATED)::
 	These are only used with the 'dcommit' and 'rebase' commands.
 +
 Passed directly to 'git rebase' when using 'dcommit' if a
@@ -769,11 +770,11 @@ option for (hopefully) obvious reasons.
 +
 This option is NOT recommended as it makes it difficult to track down
 old references to SVN revision numbers in existing documentation, bug
-reports and archives.  If you plan to eventually migrate from SVN to Git
-and are certain about dropping SVN history, consider
-linkgit:git-filter-branch[1] instead.  filter-branch also allows
-reformatting of metadata for ease-of-reading and rewriting authorship
-info for non-"svn.authorsFile" users.
+reports, and archives.  If you plan to eventually migrate from SVN to
+Git and are certain about dropping SVN history, consider
+https://github.com/newren/git-filter-repo[git-filter-repo] instead.
+filter-repo also allows reformatting of metadata for ease-of-reading
+and rewriting authorship info for non-"svn.authorsFile" users.
 
 svn.useSvmProps::
 svn-remote.<name>.useSvmProps::