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/gitcli.txt

@@ -37,6 +37,12 @@ arguments.  Here are the rules:
    file called HEAD in your work tree, `git diff HEAD` is ambiguous, and
    you have to say either `git diff HEAD --` or `git diff -- HEAD` to
    disambiguate.
+
+ * Because `--` disambiguates revisions and paths in some commands, it
+   cannot be used for those commands to separate options and revisions.
+   You can use `--end-of-options` for this (it also works for commands
+   that do not distinguish between revisions in paths, in which case it
+   is simply an alias for `--`).
 +
 When writing a script that is expected to handle random user-input, it is
 a good practice to make it explicit which arguments are which by placing
@@ -120,6 +126,11 @@ usage: git describe [<options>] <commit-ish>*
     --long                always use long format
     --abbrev[=<n>]        use <n> digits to display SHA-1s
 ---------------------------------------------
++
+Note that some subcommand (e.g. `git grep`) may behave differently
+when there are things on the command line other than `-h`, but `git
+subcmd -h` without anything else on the command line is meant to
+consistently give the usage.
 
 --help-all::
 	Some Git commands take options that are only used for plumbing or that
@@ -205,8 +216,8 @@ only affects the files in the working tree, but with
 entries, and with `--cached`, it modifies only the index
 entries.
 
-See also http://marc.info/?l=git&m=116563135620359 and
-http://marc.info/?l=git&m=119150393620273 for further
+See also https://lore.kernel.org/git/7v64clg5u9.fsf@assigned-by-dhcp.cox.net/ and
+https://lore.kernel.org/git/7vy7ej9g38.fsf@gitster.siamese.dyndns.org/ for further
 information.
 
 Some other commands that also work on files in the working tree and/or