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>

Documentation/config/core.txt

@@ -776,6 +776,10 @@ 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

@@ -13,6 +13,8 @@
 #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"
@@ -2239,6 +2241,9 @@ int cmd_fetch(int argc, const char **argv, const char *prefix)
 	}
 	string_list_remove_duplicates(&list, 0);
 
+	if (core_gvfs & 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 @@
 
 #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)
 
 void gvfs_load_config_value(const char *value);
 int gvfs_config_is_set(int mask);