Merge pull request git-for-windows#378: Scalar: Fix some flaky tests with retry logic and better GVFS Protocol awareness

I ran the Scalar functional tests at least 10 times before getting a full green build. One of the most common failures was a failure during `scalar clone` with a strange error. Looking at the TRACE2 logs, I found these entries:

```
d0 | main                     | child_start  |     |  0.159207 |           |              | [ch2] class:? argv:[git fetch --quiet origin]
d0 | main                     | child_exit   |     | 15.823433 | 15.664226 |              | [ch2] pid:16571 code:128
d0 | main                     | error        |     |           |           |              | Partial clone failed; Trying full clone
d0 | main                     | error        |     |           |           |              | could not configure for full clone
d0 | main                     | exit         |     | 15.823775 |           |              | code:1
d0 | main                     | atexit       |     | 15.823790 |           |              | code:1
```

This means that the first `git fetch` failed for some reason, and we started running `set_config()` to un-set the partial-clone config options. However, we are not in a partial clone at this state! We are working against Azure Repos.

There are two things in this PR that should help:

1. Add retry logic to `run_git()` which will retry the `git fetch` when it fails (also other subcommands).
2. Fix this use of `set_config()` by skipping it if we are using the GVFS Protocol.

Author: derrickstolee

contrib/scalar/scalar.c

@@ -107,20 +107,25 @@ static void setup_enlistment_directory(int argc, const char **argv,
 	setup_git_directory();
 }
 
+static int git_retries = 3;
+
 static int run_git(const char *arg, ...)
 {
 	struct strvec argv = STRVEC_INIT;
 	va_list args;
 	const char *p;
-	int res;
+	int res, attempts;
 
 	va_start(args, arg);
 	strvec_push(&argv, arg);
 	while ((p = va_arg(args, const char *)))
 		strvec_push(&argv, p);
 	va_end(args);
 
-	res = run_command_v_opt(argv.v, RUN_GIT_CMD);
+	for (attempts = 0, res = 1;
+	     res && attempts < git_retries;
+	     attempts++)
+		res = run_command_v_opt(argv.v, RUN_GIT_CMD);
 
 	strvec_clear(&argv);
 	return res;
@@ -879,6 +884,7 @@ static int cmd_clone(int argc, const char **argv)
 	char *cache_key = NULL, *shared_cache_path = NULL;
 	struct strbuf buf = STRBUF_INIT;
 	int res;
+	int gvfs_protocol;
 
 	argc = parse_options(argc, argv, NULL, clone_options, clone_usage, 0);
 
@@ -1006,8 +1012,10 @@ static int cmd_clone(int argc, const char **argv)
 		goto cleanup;
 	}
 
-	if (cache_server_url ||
-	    supports_gvfs_protocol(url, &default_cache_server_url)) {
+	gvfs_protocol = cache_server_url ||
+			supports_gvfs_protocol(url, &default_cache_server_url);
+
+	if (gvfs_protocol) {
 		if (!cache_server_url)
 			cache_server_url = default_cache_server_url;
 		if (set_config("core.useGVFSHelper=true") ||
@@ -1042,7 +1050,12 @@ static int cmd_clone(int argc, const char **argv)
 		return error(_("could not configure '%s'"), dir);
 
 	if ((res = run_git("fetch", "--quiet", "origin", NULL))) {
-		warning(_("Partial clone failed; Trying full clone"));
+		if (gvfs_protocol) {
+			res = error(_("failed to prefetch commits and trees"));
+			goto cleanup;
+		}
+
+		warning(_("partial clone failed; attempting full clone"));
 
 		if (set_config("remote.origin.promisor") ||
 		    set_config("remote.origin.partialCloneFilter")) {