ExternalProject: Ensure git fetch if updating to hash we don't have yet

In ac6a4d4884 (ExternalProject: Improve robustness of update step,
2020-10-17), the method used to check whether we already have a
commit or not was changed from using git rev-list to git rev-parse.
The new logic assumed rev-parse would output nothing if given a commit
hash it didn't know about, but it simply prints the hash again without
raising an error in this scenario. Amend that logic by adding ^{commit} to
the ref to ensure we do get an error if that ref is not currently known.

Fixes: #22166

Modules/ExternalProject-gitupdate.cmake.in

@@ -5,7 +5,7 @@ cmake_minimum_required(VERSION 3.5)
 
 function(get_hash_for_ref ref out_var err_var)
   execute_process(
-    COMMAND "@git_EXECUTABLE@" rev-parse "${ref}"
+    COMMAND "@git_EXECUTABLE@" rev-parse "${ref}^{commit}"
     WORKING_DIRECTORY "@work_dir@"
     RESULT_VARIABLE error_code
     OUTPUT_VARIABLE ref_hash

Tests/RunCMake/ExternalProject/FetchGitRefs.cmake

@@ -11,7 +11,7 @@ file(MAKE_DIRECTORY ${srcDir})
 file(GLOB entries ${srcRepo}/*)
 file(REMOVE_RECURSE ${entries} ${binDir})
 file(TOUCH ${srcRepo}/firstFile.txt)
-configure_file(${CMAKE_CURRENT_LIST_DIR}/FetchGitTags/CMakeLists.txt
+configure_file(${CMAKE_CURRENT_LIST_DIR}/FetchGitRefs/CMakeLists.txt
     ${srcDir}/CMakeLists.txt COPYONLY)
 
 function(execGitCommand)
@@ -63,5 +63,22 @@ execGitCommand(commit -m "Second file")
 execGitCommand(tag -a -m "Adding tag" tag_of_interest)
 execGitCommand(reset --hard HEAD~1)
 
-message(STATUS "Second configure-and-build")
+message(STATUS "Configure-and-build, update to tag")
 configureAndBuild(tag_of_interest)
+
+# Do the same, but this time for a commit hash
+file(TOUCH ${srcRepo}/thirdFile.txt)
+execGitCommand(add thirdFile.txt)
+execGitCommand(commit -m "Third file")
+execGitCommand(tag -a -m "Adding another tag" check_for_hash)
+execGitCommand(reset --hard HEAD~1)
+execute_process(
+  WORKING_DIRECTORY ${srcRepo}
+  COMMAND ${GIT_EXECUTABLE} rev-parse check_for_hash
+  COMMAND_ERROR_IS_FATAL ANY
+  OUTPUT_VARIABLE commit_hash
+  OUTPUT_STRIP_TRAILING_WHITESPACE
+)
+
+message(STATUS "Configure-and-build, update to commit hash ${commit_hash}")
+configureAndBuild(${commit_hash})

Tests/RunCMake/ExternalProject/RunCMakeTest.cmake

@@ -185,6 +185,6 @@ if(GIT_EXECUTABLE)
   # Note that there appear to be differences in where git writes its output to
   # on some platforms. It may go to stdout or stderr, so force it to be merged.
   set(RunCMake_TEST_OUTPUT_MERGE TRUE)
-  run_cmake(FetchGitTags)
+  run_cmake(FetchGitRefs)
   set(RunCMake_TEST_OUTPUT_MERGE FALSE)
 endif()