feat(git): rev completion for :Gdiffsplit family

lua/git/repo.lua

@@ -111,6 +111,41 @@ function M.head(path)
     return nil
 end
 
+---@param worktree string
+---@return string[]
+function M.list_refs(worktree)
+    local out = util.exec({
+        "git",
+        "for-each-ref",
+        "--format=%(refname:short)",
+        "refs/heads",
+        "refs/tags",
+        "refs/remotes",
+    }, { cwd = worktree, silent = true })
+    if not out then
+        return {}
+    end
+    local refs = util.split_lines(out)
+    table.insert(refs, 1, "HEAD")
+    return refs
+end
+
+---@param arg_lead string
+---@return string[]
+function M.complete_rev(arg_lead)
+    local _, worktree = M.resolve_cwd()
+    if not worktree then
+        return {}
+    end
+    local matches = {}
+    for _, ref in ipairs(M.list_refs(worktree)) do
+        if ref:sub(1, #arg_lead) == arg_lead then
+            table.insert(matches, ref)
+        end
+    end
+    return matches
+end
+
 ---@param worktree string
 ---@param rev string
 ---@param short boolean