Merge pull request #228: "gvfs-helper prefetch" during "git fetch"

This is a follow-up to #227.

1. When a new flag is added to our Git config, we can run `gvfs-helper prefetch` inside of our `git fetch` calls. This will help ensure we have updated commits and trees even if the background prefetches have fallen behind (or are not running).

2. With a new `--no-update-remote-refs` we can avoid updating the `refs/remotes` namespace. This will allow us to run `git fetch --all --no-update-remote-refs +refs/heads/*:refs/hidden/*` and we will get the new refs into a local folder (that doesn't appear anywhere). The most important thing is that users will still see when their remote refs update.

Documentation/config/core.txt

@@ -670,6 +670,10 @@ 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

@@ -27,6 +27,8 @@
 #include "branch.h"
 #include "promisor-remote.h"
 #include "commit-graph.h"
+#include "gvfs.h"
+#include "gvfs-helper-client.h"
 
 #define FORCED_UPDATES_DELAY_WARNING_IN_MS (10 * 1000)
 
@@ -1830,6 +1832,9 @@ int cmd_fetch(int argc, const char **argv, const char *prefix)
 		}
 	}
 
+	if (core_gvfs & GVFS_PREFETCH_DURING_FETCH)
+		gh_client__prefetch(0, NULL);
+
 	if (remote) {
 		if (filter_options.choice || has_promisor_remote())
 			fetch_one_setup_partial(remote);

gvfs.h

@@ -17,6 +17,7 @@
 #define GVFS_NO_DELETE_OUTSIDE_SPARSECHECKOUT       (1 << 3)
 #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);