fetch: use gvfs-helper prefetch under config

The gvfs-helper allows us to download prefetch packs using a simple
subprocess call. The gvfs-helper-client.h method will automatically
compute the timestamp if passing 0, and passing NULL for the number
of downloaded packs is valid.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>

Author: derrickstolee

Documentation/config/core.adoc

@@ -809,6 +809,10 @@ core.gvfs::
 		is first accessed and brought down to the client. Git.exe can't
 		currently tell the first access vs subsequent accesses so this
 		flag just blocks them from occurring at all.
+	GVFS_PREFETCH_DURING_FETCH::
+		Bit value 128
+		While performing a `git fetch` command, use the gvfs-helper to
+		perform a "prefetch" of commits and trees.
 --
 
 core.useGvfsHelper::

builtin/fetch.c

@@ -21,6 +21,8 @@
 #include "string-list.h"
 #include "remote.h"
 #include "transport.h"
+#include "gvfs.h"
+#include "gvfs-helper-client.h"
 #include "run-command.h"
 #include "parse-options.h"
 #include "sigchain.h"
@@ -2301,7 +2303,7 @@ static int fetch_one(struct remote *remote, int argc, const char **argv,
 int cmd_fetch(int argc,
 	      const char **argv,
 	      const char *prefix,
-	      struct repository *repo UNUSED)
+	      struct repository *repo)
 {
 	struct fetch_config config = {
 		.display_format = DISPLAY_FORMAT_FULL,
@@ -2572,6 +2574,9 @@ int cmd_fetch(int argc,
 	}
 	string_list_remove_duplicates(&list, 0);
 
+	if (gvfs_config_is_set(repo, GVFS_PREFETCH_DURING_FETCH))
+		gh_client__prefetch(0, NULL);
+
 	if (negotiate_only) {
 		struct oidset acked_commits = OIDSET_INIT;
 		struct oidset_iter iter;

gvfs.h

@@ -28,6 +28,7 @@ struct repository;
 
 #define GVFS_FETCH_SKIP_REACHABILITY_AND_UPLOADPACK (1 << 4)
 #define GVFS_BLOCK_FILTERS_AND_EOL_CONVERSIONS      (1 << 6)
+#define GVFS_PREFETCH_DURING_FETCH		    (1 << 7)
 
 int gvfs_config_is_set(struct repository *r, int mask);
 int gvfs_virtualize_objects(struct repository *r);