feat(git): replace vim-fugitive with custom git module

lua/git/repo.lua

@@ -274,12 +274,38 @@ local function head(path)
     return nil
 end
 
+---Resolve a git revision to its object SHA. Returns nil if the ref can't be
+---resolved (root-commit's `^`, blob's `^`, malformed ref, etc.). When `short`
+---is true, the result is abbreviated via `core.abbrev` (auto-extended by git
+---to keep the prefix unique in the current repo).
+---@param worktree string
+---@param ref string
+---@param short? boolean
+---@return string?
+local function rev_parse(worktree, ref, short)
+    local cmd = { "git", "rev-parse", "--verify", "--quiet" }
+    if short then
+        table.insert(cmd, "--short")
+    end
+    table.insert(cmd, ref)
+    local result = vim.system(cmd, { cwd = worktree, text = true }):wait()
+    if result.code ~= 0 then
+        return nil
+    end
+    local sha = vim.trim(result.stdout or "")
+    if sha == "" then
+        return nil
+    end
+    return sha
+end
+
 return {
     UNMERGED = UNMERGED,
     head = head,
     indicator = indicator,
     refresh_buf = refresh_buf,
     resolve = resolve,
+    rev_parse = rev_parse,
     stop_all = stop_all,
     unregister = unregister,
 }