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/gitremote-helpers.txt

@@ -238,6 +238,9 @@ the remote repository.
 	`--signed-tags=verbatim` to linkgit:git-fast-export[1].  In the
 	absence of this capability, Git will use `--signed-tags=warn-strip`.
 
+'object-format'::
+	This indicates that the helper is able to interact with the remote
+	side using an explicit hash algorithm extension.
 
 
 COMMANDS
@@ -257,12 +260,14 @@ Support for this command is mandatory.
 'list'::
 	Lists the refs, one per line, in the format "<value> <name>
 	[<attr> ...]". The value may be a hex sha1 hash, "@<dest>" for
-	a symref, or "?" to indicate that the helper could not get the
-	value of the ref. A space-separated list of attributes follows
-	the name; unrecognized attributes are ignored. The list ends
-	with a blank line.
+	a symref, ":<keyword> <value>" for a key-value pair, or
+	"?" to indicate that the helper could not get the value of the
+	ref. A space-separated list of attributes follows the name;
+	unrecognized attributes are ignored. The list ends with a
+	blank line.
 +
 See REF LIST ATTRIBUTES for a list of currently defined attributes.
+See REF LIST KEYWORDS for a list of currently defined keywords.
 +
 Supported if the helper has the "fetch" or "import" capability.
 
@@ -297,9 +302,13 @@ Supported if the helper has the "option" capability.
 	same batch are complete. Only objects which were reported
 	in the output of 'list' with a sha1 may be fetched this way.
 +
-Optionally may output a 'lock <file>' line indicating a file under
-GIT_DIR/objects/pack which is keeping a pack until refs can be
-suitably updated.
+Optionally may output a 'lock <file>' line indicating the full path of
+a file under `$GIT_DIR/objects/pack` which is keeping a pack until
+refs can be suitably updated.  The path must end with `.keep`. This is
+a mechanism to name a <pack,idx,keep> tuple by giving only the keep
+component.  The kept pack will not be deleted by a concurrent repack,
+even though its objects may not be referenced until the fetch completes.
+The `.keep` file will be deleted at the conclusion of the fetch.
 +
 If option 'check-connectivity' is requested, the helper must output
 'connectivity-ok' if the clone is self-contained and connected.
@@ -401,7 +410,9 @@ Supported if the helper has the "connect" capability.
 	trying to fall back).  After line feed terminating the positive
 	(empty) response, the output of the service starts.  Messages
 	(both request and response) must consist of zero or more
-	PKT-LINEs, terminating in a flush packet. The client must not
+	PKT-LINEs, terminating in a flush packet. Response messages will
+	then have a response end packet after the flush packet to
+	indicate the end of a response.  The client must not
 	expect the server to store any state in between request-response
 	pairs.  After the connection ends, the remote helper exits.
 +
@@ -426,6 +437,18 @@ attributes are defined.
 	This ref is unchanged since the last import or fetch, although
 	the helper cannot necessarily determine what value that produced.
 
+REF LIST KEYWORDS
+-----------------
+
+The 'list' command may produce a list of key-value pairs.
+The following keys are defined.
+
+'object-format'::
+	The refs are using the given hash algorithm.  This keyword is only
+	used if the server and client both support the object-format
+	extension.
+
+
 OPTIONS
 -------
 
@@ -505,6 +528,19 @@ set by Git if the remote helper has the 'option' capability.
 	Indicate that only the objects wanted need to be fetched, not
 	their dependents.
 
+'option atomic' {'true'|'false'}::
+	When pushing, request the remote server to update refs in a single atomic
+	transaction.  If successful, all refs will be updated, or none will.  If the
+	remote side does not support this capability, the push will fail.
+
+'option object-format' {'true'|algorithm}::
+	If 'true', indicate that the caller wants hash algorithm information
+	to be passed back from the remote.  This mode is used when fetching
+	refs.
++
+If set to an algorithm, indicate that the caller wants to interact with
+the remote side using that algorithm.
+
 SEE ALSO
 --------
 linkgit:git-remote[1]