feat(repo): sync forks on the server, wire forks into clones, validate set-default

repo sync OWNER/REPO, run where no Git remote points to the repository,
now updates it on the server: a fork's branch fast-forwards to its
parent's through merge-upstream, and a pull mirror fetches its source.

repo fork reports an existing fork ("! bob/x already exists") instead of
a raw HTTP 409, refuses to fork a repository into its own owner, and
gains --fork-name, --clone (Git flags after --) and --remote, which adds
the fork as origin and renames the old origin to upstream.

repo clone passes Git flags after --, and takes --upstream-remote-name
and --no-upstream for forks. Clones report what they set up.

repo set-default checks that the repository exists before saving it,
prints the saved repository with --view, removes it with --unset, and
reports the change.

Signed-off-by: bircni <bircni@icloud.com>

internal/app/args_test.go

@@ -91,7 +91,7 @@ func TestArgumentErrorsShowTheUsageLine(t *testing.T) {
 		{[]string{"pr", "checkout"}, "missing pull request number\nUsage: tea pr checkout <number> [flags]"},
 		{[]string{"run", "view"}, "missing run ID\nUsage: tea run view [<run-id>] [flags]"},
 		{[]string{"release", "asset", "download", "v1"}, "missing asset ID\nUsage: tea release asset download <tag> <asset-id> [flags]"},
-		{[]string{"repo", "clone"}, "missing repository\nUsage: tea repo clone <repository> [directory] [flags]"},
+		{[]string{"repo", "clone"}, "missing repository\nUsage: tea repo clone <repository> [<directory>] [-- <gitflags>...] [flags]"},
 	} {
 		requireUsageBeforeRequests(t, tc.want, tc.args...)
 	}