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/config/http.txt

@@ -29,6 +29,27 @@ http.proxyAuthMethod::
 * `ntlm` - NTLM authentication (compare the --ntlm option of `curl(1)`)
 --
 
+http.proxySSLCert::
+	The pathname of a file that stores a client certificate to use to authenticate
+	with an HTTPS proxy. Can be overridden by the `GIT_PROXY_SSL_CERT` environment
+	variable.
+
+http.proxySSLKey::
+	The pathname of a file that stores a private key to use to authenticate with
+	an HTTPS proxy. Can be overridden by the `GIT_PROXY_SSL_KEY` environment
+	variable.
+
+http.proxySSLCertPasswordProtected::
+	Enable Git's password prompt for the proxy SSL certificate.  Otherwise OpenSSL
+	will prompt the user, possibly many times, if the certificate or private key
+	is encrypted. Can be overriden by the `GIT_PROXY_SSL_CERT_PASSWORD_PROTECTED`
+	environment variable.
+
+http.proxySSLCAInfo::
+	Pathname to the file containing the certificate bundle that should be used to
+	verify the proxy with when using an HTTPS proxy. Can be overriden by the
+	`GIT_PROXY_SSL_CAINFO` environment variable.
+
 http.emptyAuth::
 	Attempt authentication without seeking a username or password.  This
 	can be used to attempt GSS-Negotiate authentication without specifying
@@ -71,7 +92,7 @@ http.saveCookies::
 http.version::
 	Use the specified HTTP protocol version when communicating with a server.
 	If you want to force the default. The available and default version depend
-	on libcurl. Actually the possible values of
+	on libcurl. Currently the possible values of
 	this option are:
 
 	- HTTP/2
@@ -84,7 +105,7 @@ http.sslVersion::
 	particular configuration of the crypto library in use. Internally
 	this sets the 'CURLOPT_SSL_VERSION' option; see the libcurl
 	documentation for more details on the format of this option and
-	for the ssl version supported. Actually the possible values of
+	for the ssl version supported. Currently the possible values of
 	this option are:
 
 	- sslv2
@@ -199,6 +220,14 @@ http.postBuffer::
 	Transfer-Encoding: chunked is used to avoid creating a
 	massive pack file locally.  Default is 1 MiB, which is
 	sufficient for most requests.
++
+Note that raising this limit is only effective for disabling chunked
+transfer encoding and therefore should be used only where the remote
+server or a proxy only supports HTTP/1.0 or is noncompliant with the
+HTTP standard.  Raising this is not, in general, an effective solution
+for most push problems, but can increase memory consumption
+significantly since the entire buffer is allocated even for small
+pushes.
 
 http.lowSpeedLimit, http.lowSpeedTime::
 	If the HTTP transfer speed is less than 'http.lowSpeedLimit'