feat(git/repo): add Repo:head_sha for cached HEAD blob lookups

lua/git/core/repo.lua

@@ -143,6 +143,7 @@ function Repo:_invalidate(relpath)
     end
     if affects_resolve(relpath) then
         self:_clear_cache_prefix("resolve:")
+        self:_clear_cache_prefix("head_blob:")
     end
     if relpath == "index" then
         self:_clear_cache_prefix("index:")
@@ -605,6 +606,15 @@ function Repo:index_sha(rel)
     return sha or nil
 end
 
+---@param rel string  worktree-relative path
+---@return string?
+function Repo:head_sha(rel)
+    local sha = self:get_cached("head_blob:" .. rel, function(self)
+        return self:rev_parse("HEAD:" .. rel, false) or false
+    end)
+    return sha or nil
+end
+
 ---@alias ow.Git.Repo.ResolveStatus "ok"|"ambiguous"|"missing"
 
 ---@param abbrev string

test/git/repo_test.lua

@@ -97,6 +97,25 @@ t.test("index_sha cache clears when the index is written", function()
     wait_cleared(r, "index:a", 2000)
 end)
 
+t.test("head_sha returns the blob sha and caches it", function()
+    local dir = h.make_repo({ a = "x\n" })
+    local r = assert(require("git.core.repo").resolve(dir))
+    local sha = r:head_sha("a")
+    t.truthy(sha and #sha > 0, "head_sha returns the HEAD blob sha")
+    t.truthy(r._cache["head_blob:a"] ~= nil, "the result is cached")
+    t.eq(r:head_sha("a"), sha, "a cached call returns the same sha")
+end)
+
+t.test("head_sha cache clears when HEAD moves", function()
+    local dir = h.make_repo({ a = "x\n" })
+    local r = assert(require("git.core.repo").resolve(dir))
+    r:head_sha("a")
+    t.truthy(r._cache["head_blob:a"] ~= nil, "sha is cached before the commit")
+    t.write(dir, "a", "y\n")
+    h.git(dir, "commit", "-aqm", "change")
+    wait_cleared(r, "head_blob:a", 2000)
+end)
+
 t.test("cache clears after top-level .git change (commit)", function()
     local dir = h.make_repo({ a = "x" })
     local r = assert(require("git.core.repo").resolve(dir))