cmake: add reftable ref storage support to GetGitRevisionDescription

The script assumed Git's traditional loose refs or packed-refs format.
When cloning with extensions.refstorage=reftable, neither .git/refs/*
nor .git/packed-refs exist, causing cmake configuration to fail.

Fall back to git rev-parse when file-based ref lookup fails.

cmake/GetGitRevisionDescription.cmake

@@ -164,6 +164,22 @@ function(get_git_head_revision _refspecvar _hashvar)
                    "${GIT_DATA}/grabRef.cmake" @ONLY)
     include("${GIT_DATA}/grabRef.cmake")
 
+    # Fallback for reftable or other storage formats
+    if(NOT HEAD_HASH OR HEAD_HASH STREQUAL "")
+        find_package(Git QUIET)
+        if(GIT_FOUND)
+            execute_process(
+                COMMAND "${GIT_EXECUTABLE}" rev-parse HEAD
+                WORKING_DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}"
+                RESULT_VARIABLE res
+                OUTPUT_VARIABLE HEAD_HASH
+                ERROR_QUIET OUTPUT_STRIP_TRAILING_WHITESPACE)
+            if(NOT res EQUAL 0)
+                set(HEAD_HASH "")
+            endif()
+        endif()
+    endif()
+
     set(${_refspecvar}
         "${HEAD_REF}"
         PARENT_SCOPE)

cmake/GetGitRevisionDescription.cmake.in

@@ -25,19 +25,21 @@ if(HEAD_CONTENTS MATCHES "ref")
 	string(REPLACE "ref: " "" HEAD_REF "${HEAD_CONTENTS}")
 	if(EXISTS "@GIT_DIR@/${HEAD_REF}")
 		configure_file("@GIT_DIR@/${HEAD_REF}" "@GIT_DATA@/head-ref" COPYONLY)
-	else()
+	elseif(EXISTS "@GIT_DIR@/packed-refs")
 		configure_file("@GIT_DIR@/packed-refs" "@GIT_DATA@/packed-refs" COPYONLY)
 		file(READ "@GIT_DATA@/packed-refs" PACKED_REFS)
 		if(${PACKED_REFS} MATCHES "([0-9a-z]*) ${HEAD_REF}")
 			set(HEAD_HASH "${CMAKE_MATCH_1}")
 		endif()
+	elseif(EXISTS "@GIT_DIR@/reftable/tables.list")
+		configure_file("@GIT_DIR@/reftable/tables.list" "@GIT_DATA@/reftable-tables.list" COPYONLY)
 	endif()
 else()
 	# detached HEAD
 	configure_file("@GIT_DIR@/HEAD" "@GIT_DATA@/head-ref" COPYONLY)
 endif()
 
-if(NOT HEAD_HASH)
+if(NOT HEAD_HASH AND EXISTS "@GIT_DATA@/head-ref")
 	file(READ "@GIT_DATA@/head-ref" HEAD_HASH LIMIT 1024)
 	string(STRIP "${HEAD_HASH}" HEAD_HASH)
 endif()