Prune remote refs to the old default branch

I've learned how to provide the refspce and, hopefully, this won't be slow even on repos with lots of remote refs

Author: jennybc

R/git-default-branch.R

@@ -405,6 +405,9 @@ rediscover_default_branch <- function(old_name = NULL, report_on_source = TRUE)
   # git branch -u origin/NEW-BRANCH-NAME NEW-BRANCH-NAME
   # git remote set-head origin -a
 
+  # optionally
+  # git remote prune origin
+
   # Note: they are assuming the relevant repo is known as origin, but it could
   # just as easily be, e.g., upstream.
 
@@ -427,7 +430,6 @@ rediscover_default_branch <- function(old_name = NULL, report_on_source = TRUE)
   # for very stale repos, a fetch is a necessary pre-requisite
   # I provide `refspec = db` to avoid fetching all refs, which can be VERY slow
   # for a repo like ggplot2 (several minutes, with no progress reporting)
-  # however this means I can't do `prune = TRUE` to prune, e.g. origin/master
   gert::git_fetch(remote = tr$name, refspec = db, verbose = FALSE, repo = repo)
   gert::git_remote_ls(remote = tr$name, verbose = FALSE, repo = repo)
 
@@ -447,6 +449,16 @@ rediscover_default_branch <- function(old_name = NULL, report_on_source = TRUE)
     repo = repo
   )
 
+  # goal: get rid of old remote tracking branch, e.g. origin/master
+  # goal, in Git-speak: git remote prune origin
+  # I provide a refspec to avoid fetching all refs, which can be VERY slow
+  # for a repo like ggplot2 (several minutes, with no progress reporting)
+  gert::git_fetch(
+    remote = tr$name,
+    refspec = glue("refs/heads/{old_name}:refs/remotes/{tr$name}/{old_name}"),
+    verbose = FALSE, repo = repo, prune = TRUE
+  )
+
   # for "ours" and "theirs", the source repo is the only remote on our radar and
   # we're done ingesting the default branch from the source repo
   # but for "fork", we also need to update
@@ -461,9 +473,13 @@ rediscover_default_branch <- function(old_name = NULL, report_on_source = TRUE)
         from = old_name_fork,
         new_name = db
       )
-      # giving refspec has same pros and cons as noted above for source repo
       gert::git_fetch(remote = "origin", refspec = db, verbose = FALSE, repo = repo)
       gert::git_remote_ls(remote = "origin", verbose = FALSE, repo = repo)
+      gert::git_fetch(
+        remote = "origin",
+        refspec = glue("refs/heads/{old_name}:refs/remotes/origin/{old_name}"),
+        verbose = FALSE, repo = repo, prune = TRUE
+      )
     }
   }