✨ Enable building mold with git reftable backend

lib/update-git-hash.cmake

@@ -6,7 +6,37 @@ if(EXISTS "${SOURCE_DIR}/.git/HEAD")
 
   if(HASH MATCHES "^ref: (.*)")
     set(HEAD "${CMAKE_MATCH_1}")
-    if(EXISTS "${SOURCE_DIR}/.git/${HEAD}")
+    if(HEAD STREQUAL "refs/heads/.invalid")
+      # Git v2.45+ with config `feature.experimental`=true, config `extensions.refstorage`=reftable,
+      # or `--ref-format=reftable`, uses the new binary reftable format, which does not expose
+      # plaintext files and breaks our no-Git wishes as the HEAD hash is not directly stored.
+      #
+      # Provide an option to disable Git and fall back to no hash if desired. Else, best-effort
+      # attempt to find ans use Git.
+      option(MOLD_NO_GIT "Do not look for Git if the new reftable format is in use" OFF)
+      if(MOLD_NO_GIT)
+        set(HASH "")
+      else()
+        find_package(Git QUIET)
+        if (NOT Git_FOUND)
+          message(WARNING "In a git repo with a reftable format, but could not find Git. No hash will be embedded in the binary.")
+          set(HASH "")
+        else()
+          execute_process(
+            COMMAND ${GIT_EXECUTABLE} rev-parse HEAD
+            OUTPUT_VARIABLE git_output
+            RESULT_VARIABLE git_result
+            ERROR_QUIET
+            OUTPUT_STRIP_TRAILING_WHITESPACE
+          )
+          if(git_result EQUAL 0)
+            set(HASH "${git_output}")
+          else()
+            message(WARNING "Git returned ${git_result} when querying the HEAD revision")
+          endif()
+        endif()
+      endif()
+    elseif(EXISTS "${SOURCE_DIR}/.git/${HEAD}")
       file(READ "${SOURCE_DIR}/.git/${HEAD}" HASH)
       string(STRIP "${HASH}" HASH)
     else()