Merge branch 'jn/experimental-opts-into-proto-v2'

"feature.experimental" configuration variable is to let volunteers
easily opt into a set of newer features, which use of the v2
transport protocol is now a part of.

* jn/experimental-opts-into-proto-v2:
  config: let feature.experimental imply protocol.version=2

Documentation/config/feature.txt

@@ -22,6 +22,10 @@ existing commit-graph file(s). Occasionally, these files will merge and the
 write may take longer. Having an updated commit-graph file helps performance
 of many Git commands, including `git merge-base`, `git push -f`, and
 `git log --graph`.
++
+* `protocol.version=2` speeds up fetches from repositories with many refs by
+allowing the client to specify which refs to list before the server lists
+them.
 
 feature.manyFiles::
 	Enable config options that optimize for repos with many files in the

Documentation/config/protocol.txt

@@ -48,7 +48,8 @@ protocol.version::
 	If set, clients will attempt to communicate with a server
 	using the specified protocol version.  If the server does
 	not support it, communication falls back to version 0.
-	If unset, the default is `0`.
+	If unset, the default is `0`, unless `feature.experimental`
+	is enabled, in which case the default is `2`.
 	Supported versions:
 +
 --

protocol.c

@@ -17,6 +17,7 @@ static enum protocol_version parse_protocol_version(const char *value)
 enum protocol_version get_protocol_version_config(void)
 {
 	const char *value;
+	int val;
 	const char *git_test_k = "GIT_TEST_PROTOCOL_VERSION";
 	const char *git_test_v;
 
@@ -30,6 +31,9 @@ enum protocol_version get_protocol_version_config(void)
 		return version;
 	}
 
+	if (!git_config_get_bool("feature.experimental", &val) && val)
+		return protocol_v2;
+
 	git_test_v = getenv(git_test_k);
 	if (git_test_v && *git_test_v) {
 		enum protocol_version env = parse_protocol_version(git_test_v);