Add kludge to deal with the appearance of forge-forgejo.el

"package.el" does not compile packages in a clean environment, so if
a package is already loaded when it is being updated, all kinds of
breakage can happen (including but by far not limited to the current
issue).

Forge also does a nasty thing; "forge.el" first `provide's its feature
and then `require's the various back-end libraries, including the newly
added "forge-forgejo.el".  This makes it possible to (require 'forge)
in the back-end libraries, and at the same time for other libraries to
depend on "forge.el" taking care of loading the back-end libraries.

If `forge' is already loaded then (require 'forge) does nothing when a
library that contains that is being compiled.  That also means that the
newly added (require 'forge-forgejo) in that library is not evaluated.
"forge-commands.el" contains code that errors at compile-time if a
referenced type is unknown, `forge-forgejo-repository' in this case.

We inject an additional (require 'forge-forgejo) in "forge-commands.el"
to make sure the type is known during compilation, even when upgrading
from a Forge version that didn't have that library and that version is
already loaded in the current Emacs session.

Closes #743.

lisp/forge-commands.el

@@ -448,6 +448,9 @@ commit, and for a file."
 (cl-defmethod forge-get-url ((notify forge-notification))
   (oref notify url))
 
+ ;; Transitional kludge for the infamous package.el defect.
+(require 'forge-forgejo)
+
 (cl-defmethod forge-format-blob-lines ((repo forge-repository) line end)
   (cl-etypecase repo ;Third-party classes require separate methods.
     ((or forge-github-repository