refactor(git): URI scheme is now a git revspec, loaded via cat-file -p

lua/git/diff.lua

@@ -57,32 +57,31 @@ local function attach_index_writer(buf, worktree, path)
     })
 end
 
----Return a buffer holding the content at `<ref>:<path>`. `ref` is
----"index", "HEAD", or a commit revspec.
+---Return a buffer holding the content addressed by a git revspec. The
+---URI is `git://<revspec>` and BufReadCmd loads via `git cat-file -p`.
 ---@param worktree string
----@param ref string
----@param path string
+---@param revspec string  any revspec git understands (e.g. `HEAD:foo`, `:foo`, `:1:foo`, `<sha>`, `<sha>:foo`)
 ---@return integer
-function M.git_show_buf(worktree, ref, path)
-    local name = "git://" .. ref .. "//" .. path
+function M.git_show_buf(worktree, revspec)
+    local name = "git://" .. revspec
     local buf = vim.fn.bufadd(name)
     vim.b[buf].git_worktree = worktree
     vim.fn.bufload(buf)
     return buf
 end
 
----BufReadCmd handler for `git://<ref>//<path>` URIs. Worktree comes from
----`vim.b[buf].git_worktree` if set, else from cwd. Ref of "index" maps
----to `git show :<path>`; "worktree" leaves the buffer empty (placeholder
----for missing files); anything else is a revspec.
+---BufReadCmd handler for `git://<revspec>` URIs. Loads content via
+---`git cat-file -p <revspec>`. Worktree comes from `vim.b[buf]
+---.git_worktree` if set, else from cwd. Index entries (revspec form
+---`:<path>` for stage 0) are made writable via `attach_index_writer`,
+---so `:w` updates the index. Other revspecs are read-only.
 ---@param buf integer
 function M.read_uri(buf)
     local name = vim.api.nvim_buf_get_name(buf)
-    local ref, path = name:match("^git://(.-)//(.*)$")
-    if not ref or path == "" then
+    local revspec = name:match("^git://(.+)$")
+    if not revspec then
         return
     end
-    ---@cast path -nil
 
     local worktree = vim.b[buf].git_worktree or select(2, repo.resolve_cwd())
     if not worktree then
@@ -94,27 +93,26 @@ function M.read_uri(buf)
     vim.bo[buf].swapfile = false
     vim.bo[buf].bufhidden = "wipe"
 
-    if ref == "worktree" then
-        vim.bo[buf].buftype = "nofile"
-        vim.bo[buf].modifiable = false
-        vim.bo[buf].modified = false
-        vim.api.nvim_exec_autocmds("BufReadPost", { buffer = buf })
-        return
-    end
-
-    local revspec = ref == "index" and (":" .. path) or (ref .. ":" .. path)
-    local stdout = util.exec({ "git", "show", revspec }, { cwd = worktree })
+    local stdout = util.exec(
+        { "git", "cat-file", "-p", revspec },
+        { cwd = worktree }
+    )
     if stdout then
         vim.api.nvim_buf_set_lines(buf, 0, -1, false, util.split_lines(stdout))
     end
 
-    if ref == "index" then
+    -- Stage-0 index entries (`:<path>` with no further `:`) are
+    -- editable; `:w` rewrites the index entry via `attach_index_writer`.
+    -- Anything else (HEAD:, <sha>:, :1:, :2:, :3:, bare object refs)
+    -- is read-only.
+    local index_path = revspec:match("^:([^:]+)$")
+    if index_path then
         vim.bo[buf].buftype = "acwrite"
         -- Re-running BufReadCmd (e.g. on `:edit`) would otherwise stack
         -- another BufWriteCmd on the same buffer, so each `:w` runs
         -- hash-object + update-index N times.
         if not vim.b[buf].git_index_writer then
-            attach_index_writer(buf, worktree, path)
+            attach_index_writer(buf, worktree, index_path)
             vim.b[buf].git_index_writer = true
         end
     else
@@ -221,8 +219,9 @@ function M.split(opts)
         return
     end
 
-    local label = opts.ref == "" and "index" or opts.ref
-    local uri = "git://" .. label .. "//" .. rel
+    -- Stage 0 (index) is `:<path>`; named refs are `<ref>:<path>`.
+    local revspec = opts.ref == "" and (":" .. rel) or (opts.ref .. ":" .. rel)
+    local uri = "git://" .. revspec
     -- Stash the worktree on the buffer so the BufReadCmd handler doesn't
     -- fall back to cwd resolution (wrong when cwd != worktree).
     local buf = vim.fn.bufadd(uri)