[bug#50426] [PATCH] import: elpa: Don't hardcode default branch to 'master'.

[Xinglu Chen <public@yoctocell.xyz>]

Otherwise, remotes without a branch named 'master' will cause an error when
importing.

* guix/import/elpa (git-repository->origin): Fallback to HEAD instead of the
  'master' branch.
---
For example, try to import the ‘consult’ package from MELPA:

--8<---------------cut here---------------start------------->8---
$ ./pre-inst-env guix import elpa -a melpa consult
;;; note: source file /home/yoctocell/src/guix/guix/import/elpa.scm
;;;       newer than compiled /home/yoctocell/src/guix/guix/import/elpa.go
;;; note: source file /home/yoctocell/src/guix/guix/import/elpa.scm
;;;       newer than compiled /home/yoctocell/.config/guix/current/lib/guile/3.0/site-ccache/guix/import/elpa.go
;;; note: source file /home/yoctocell/src/guix/guix/import/elpa.scm
;;;       newer than compiled /home/yoctocell/.guix-home/profile/lib/guile/3.0/site-ccache/guix/import/elpa.go
Backtrace:
In ice-9/boot-9.scm:
  1752:10 16 (with-exception-handler _ _ #:unwind? _ #:unwind-for-type _)
In unknown file:
          15 (apply-smob/0 #<thunk 7f9dcef69f60>)
In ice-9/boot-9.scm:
    724:2 14 (call-with-prompt _ _ #<procedure default-prompt-handler (k proc)>)
In ice-9/eval.scm:
    619:8 13 (_ #(#(#<directory (guile-user) 7f9dcef63c80>)))
In guix/ui.scm:
   2185:7 12 (run-guix . _)
  2148:10 11 (run-guix-command _ . _)
In guix/scripts/import.scm:
   121:11 10 (guix-import . _)
In guix/scripts/import/elpa.scm:
   104:23  9 (guix-import-elpa . _)
In guix/import/elpa.scm:
    207:2  8 (elpa->guix-package _ #:repo _ #:version _)
In ice-9/boot-9.scm:
  1752:10  7 (with-exception-handler _ _ #:unwind? _ #:unwind-for-type _)
In guix/store.scm:
   658:37  6 (thunk)
In guix/git.scm:
    481:8  5 (latest-repository-commit #<store-connection 256.99 7f9dc996c820> "https://github.com/…" …)
    247:4  4 (update-cached-checkout _ #:ref _ #:recursive? _ #:check-out? _ #:starting-commit _ # _ …)
   211:19  3 (resolve _)
In git/branch.scm:
    101:8  2 (_ _ _ _)
In git/bindings.scm:
     77:2  1 (raise-git-error _)
In ice-9/boot-9.scm:
  1685:16  0 (raise-exception _ #:continuable? _)

ice-9/boot-9.scm:1685:16: In procedure raise-exception:
Git error: cannot locate remote-tracking branch 'origin/master'
--8<---------------cut here---------------end--------------->8---

 guix/import/elpa.scm | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/guix/import/elpa.scm b/guix/import/elpa.scm
index c0dc5acf51..f9689aaa13 100644
--- a/guix/import/elpa.scm
+++ b/guix/import/elpa.scm
@@ -4,6 +4,7 @@
 ;;; Copyright © 2018 Oleg Pykhalov <go.wigust@gmail.com>
 ;;; Copyright © 2020 Martin Becze <mjbecze@riseup.net>
 ;;; Copyright © 2020 Ricardo Wurmus <rekado@elephly.net>
+;;; Copyright © 2021 Xinglu Chen <public@yoctocell.xyz>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -257,7 +258,7 @@ RECIPE."
      ((assoc-ref recipe #:commit)
       => (lambda (commit) (cons 'commit commit)))
      (else
-      '(branch . "master"))))
+      '())))
 
   (let-values (((directory commit) (download-git-repository url ref)))
     `(origin

base-commit: 9540323458de87b0b8aa421e449a4fe27af7c393
-- 
2.33.0