connected: add incremental connectivity check

Documentation/config/transfer.adoc

@@ -1,3 +1,25 @@
+transfer.connectivityCheck::
+	Choose which algorithm to use for the connectivity check
+	performed during object transfer operations such as
+	linkgit:git-fetch[1] and linkgit:git-receive-pack[1].
+	The connectivity check verifies that all objects reachable
+	from the incoming tips are available locally or, in a partial
+	clone, promised by a promisor remote.
+	The variants are as follows:
++
+--
+`rev-list` (default);;
+	Delegate to `rev-list --objects --not --all`.  This walks
+	the full object closure of the boundary commits.
+`incremental`;;
+	Verify incoming commits by diffing their trees against parent
+	trees, recursively descending only into entries that differ.
+	The largest benefits occur when incoming commits change a
+	small fraction of a large tree closure.
+	Falls back to `rev-list` when replacement objects are
+	active or a deepening fetch is in progress.
+--
+
 transfer.credentialsInUrl::
 	A configured URL can contain plaintext credentials in the form
 	`<protocol>://<user>:<password>@<domain>/<path>`. You may want

Makefile

@@ -812,6 +812,7 @@ TEST_BUILTINS_OBJS += test-bitmap.o
 TEST_BUILTINS_OBJS += test-bloom.o
 TEST_BUILTINS_OBJS += test-bundle-uri.o
 TEST_BUILTINS_OBJS += test-cache-tree.o
+TEST_BUILTINS_OBJS += test-check-connected.o
 TEST_BUILTINS_OBJS += test-chmtime.o
 TEST_BUILTINS_OBJS += test-config.o
 TEST_BUILTINS_OBJS += test-crontab.o