fix: support finding pr branch via git config branch keys (#806)

lua/octo/utils.lua

@@ -449,9 +449,14 @@ end
 --- Determines if we are locally are in a branch matching the pr head ref
 --- @param pr PullRequest
 --- @return boolean
-function M.in_pr_branch(pr)
+function M.in_pr_branch_locally_tracked(pr)
   local cmd = "git rev-parse --abbrev-ref --symbolic-full-name @{u}"
-  local local_branch_with_local_remote = vim.split(string.gsub(vim.fn.system(cmd), "%s+", ""), "/")
+  local cmd_out = vim.fn.system(cmd)
+  if vim.v.shell_error ~= 0 then
+    return false
+  end
+
+  local local_branch_with_local_remote = vim.split(string.gsub(cmd_out, "%s+", ""), "/")
   local local_remote = local_branch_with_local_remote[1]
   local local_branch = table.concat(local_branch_with_local_remote, "/", 2)
 
@@ -465,6 +470,73 @@ function M.in_pr_branch(pr)
   return false
 end
 
+-- the gh cli stores the remote info for a PR branch in a branch specific
+-- config value.
+--
+-- this function searches for the upstream info, if it exists, for the current
+-- checked out branch, within git config branch specific values.
+--
+-- if a config value is found the upstream branch name is returned, verbatim.
+-- if no config value is found an empty string is returned.
+--
+-- this is useful when you want to determine if the currently checked out branch
+-- maps to a PR HEAD, when 'gh pr checkout' is used.
+function M.get_upstream_branch_from_config()
+  local branch_cmd = "git rev-parse --abbrev-ref HEAD"
+  local branch = vim.fn.system(branch_cmd)
+  if vim.v.shell_error ~= 0 then
+    return ""
+  end
+
+  if #branch == 0 then
+    return ""
+  end
+
+  -- trim white space off branch
+  branch = string.gsub(branch, "%s+", "")
+
+  local merge_config_cmd = string.format('git config --get-regexp "^branch\\.%s\\.merge"', branch)
+
+  local merge_config = vim.fn.system(merge_config_cmd)
+  if vim.v.shell_error ~= 0 then
+    return ""
+  end
+
+  if #merge_config == 0 then
+    return ""
+  end
+
+  -- split merge_config to key, value with space delimeter
+  local merge_config_kv = vim.split(merge_config, "%s+")
+  -- use > 2 since there maybe some garbage white space at the end of the map.
+  if #merge_config_kv < 2 then
+    return ""
+  end
+
+  local upstream_branch_ref = merge_config_kv[2]
+
+  -- remove the prefix /refs/heads/ from upstream_branch_ref resulting in
+  -- branch's name.
+  local upstream_branch_name = string.gsub(upstream_branch_ref, "^refs/heads/", "")
+
+  return upstream_branch_name
+end
+
+-- Determines if we are locally in a branch matting the pr head ref when
+-- the remote and branch information is stored in the branch's git config values
+-- The gh CLI tool stores remote info directly in {branch.{branch}.x} configuration
+-- fields and does not create a remote
+function M.in_pr_branch_config_tracked(pr)
+  return M.get_upstream_branch_from_config():lower() == pr.head_ref_name
+end
+
+--- Determines if we are locally are in a branch matching the pr head ref
+--- @param pr PullRequest
+--- @return boolean
+function M.in_pr_branch(pr)
+  return M.in_pr_branch_locally_tracked(pr) or M.in_pr_branch_config_tracked(pr)
+end
+
 function M.checkout_pr(pr_number)
   gh.run {
     args = { "pr", "checkout", pr_number },