fix(DiffScreen): Reduce git commands by 60% when opening diff view (#429)

* feat(git_repo): Cache discover() result per directory

Cache the result of git rev-parse --show-toplevel per directory. The
repo root for a given path never changes during a session, so we can
safely cache it after the first lookup. This eliminates a git command on
every diff view open and other operations that need to discover the
repo.

* fix: Gate conflict checks with filesystem check

Skip expensive per-file conflict checks (git ls-files -u and git status)
when not in a merge/rebase/cherry-pick state. Use git_conflict.status()
which only checks for files like .git/MERGE_HEAD - fast filesystem
checks, no git commands.

In the common case (no active merge), this eliminates 2 git commands
when opening a diff view.

Author: MaxFangX

lua/vgit/features/screens/DiffScreen/Model.lua

@@ -68,28 +68,35 @@ function Model:fetch(filename)
 
   self.git_file = GitFile(filename)
 
-  local has_conflict = self.git_file:has_conflict()
-  if has_conflict and self:is_staged() then
-    self.state.diff = nil
-    return
+  -- Fast filesystem check: are we in a merge/rebase/cherry-pick state?
+  -- Only run expensive per-file conflict checks if so.
+  local conflict_status = git_conflict.status(self.git_file.reponame)
+  if conflict_status then
+    local has_conflict = self.git_file:has_conflict()
+    if has_conflict and self:is_staged() then
+      self.state.diff = nil
+      return
+    end
+
+    local status = self.git_file:status()
+    if status and status:is_unmerged() then
+      local lines, lines_err = self:get_lines(filename)
+      if lines_err then return nil, lines_err end
+
+      local layout_type = self:get_layout_type()
+      local conflicts = git_conflict.parse(lines)
+      self.state.diff = Diff():generate(nil, lines, layout_type, { conflicts = conflicts })
+      return self.state.diff
+    end
   end
 
-  local status = self.git_file:status()
-
   local lines, lines_err = self:get_lines(filename)
   if lines_err then return nil, lines_err end
 
-  local layout_type = self:get_layout_type()
-  if status and status:is_unmerged() then
-    local conflicts = git_conflict.parse(lines)
-    self.state.diff = Diff():generate(nil, lines, layout_type, { conflicts = conflicts })
-    return self.state.diff
-  end
   local hunks, hunks_err = self:get_hunks(lines)
   if hunks_err then return nil, hunks_err end
 
-  self.state.diff = Diff():generate(hunks, lines, layout_type)
-
+  self.state.diff = Diff():generate(hunks, lines, self:get_layout_type())
   return self.state.diff
 end
 

lua/vgit/git/git_repo.lua

@@ -2,17 +2,26 @@ local gitcli = require('vgit.git.gitcli')
 
 local git_repo = {}
 
+-- Cache: directory path -> repo root. Stable within a session; only stale if
+-- user runs `git init` in a new location (rare, restart neovim to clear).
+local discover_cache = {}
+
 function git_repo.config(reponame)
   if not reponame then return nil, { 'reponame is required' } end
   return gitcli.run({ '-C', reponame, 'config', '--list' })
 end
 
 function git_repo.discover(filepath)
   local dirname = (filepath and vim.fn.fnamemodify(filepath, ':p:h')) or vim.loop.cwd()
-  local result, err = gitcli.run({ '-C', dirname, 'rev-parse', '--show-toplevel' })
 
+  local cached = discover_cache[dirname]
+  if cached then return cached end
+
+  local result, err = gitcli.run({ '-C', dirname, 'rev-parse', '--show-toplevel' })
   if err then return nil, err end
-  if #result==0 then return nil, {} end
+  if #result == 0 then return nil, {} end
+
+  discover_cache[dirname] = result[1]
   return result[1]
 end