raco pkg: another attempt to fix git commands for an update

Document and and exploit that any fragment in the Git or GitHub URL
for a package source must name a branch or tag (as opposed to a
commit) to work with clone linking.

pkgs/racket-doc/pkg/scribblings/git-workflow.scrbl

@@ -42,7 +42,10 @@ repository source, the package is installed by creating a Git clone of
 @nonterm{git-pkg-source} as @nonterm{dir}. The clone's checkout is
 linked in the same way as a directory, but unlike a plain directory
 link, the Racket package manager keeps track of the repository
-connection.
+connection. The @nonterm{git-pkg-source} must be a Git or GitHub
+package source, or it must be a package name that the catalog maps to
+a Git or GitHub package source; if the source URL includes a fragment,
+it must name a branch or tag (as opposed to a raw commit).
 
 When the repository at @nonterm{git-pkg-source} is changed so that the
 source has a new checksum, then @command-ref{update} for the package pulls

racket/collects/pkg/private/stage.rkt

@@ -166,7 +166,7 @@
          (parameterize ([current-directory clone-dir])
            (download-printf "Fetching from remote repository ~a\n"
                             pkg-no-query)
-           (git #:status status "fetch" pkg-no-query)))
+           (git #:status status "fetch" pkg-no-query branch)))
 
        (cond
         [tmp-dir