init-repository: Fix passing no --alternates option

Commit d6918d7ac5 broke init-repository if
you did not pass an --alternates option.

Pick-to: 6.8
Task-number: QTBUG-142712
Fixes: QTBUG-142744
Change-Id: Ia727b420a34a46e52f0f05e549aabe2c975c6d7e
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
(cherry picked from commit 6bd3b23b6c)
(cherry picked from commit f1d54f955c)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

cmake/QtIRCommandLineHelpers.cmake

@@ -421,3 +421,19 @@ function(qt_ir_get_option_as_cmake_flag_option cli_name cmake_option_name out_va
     endif()
     set(${out_var} "${cmake_option}" PARENT_SCOPE)
 endfunction()
+
+# Get the value of a command line option as existing absolute path.
+# Yield error if the path does not exist.
+# Convert to absolute path if necessary.
+function(qt_ir_get_option_as_existing_absolute_path name value)
+    qt_ir_get_option_value("${name}" path)
+    if(NOT "${path}" STREQUAL "")
+        if(NOT EXISTS "${path}")
+            qt_ir_add_error("The path '${path}' passed with -${name} does not exist.")
+        endif()
+        if(NOT IS_ABSOLUTE "${path}")
+            get_filename_component(path "${path}" ABSOLUTE)
+        endif()
+    endif()
+    set("${value}" "${path}" PARENT_SCOPE)
+endfunction()

cmake/QtIRHelpers.cmake

@@ -295,7 +295,7 @@ function(qt_ir_run_after_args_parsed top_level_src_path out_var_exit_reason)
         "${working_directory}")
 
     # Get some additional options to pass down.
-    qt_ir_get_option_value(alternates alternates)
+    qt_ir_get_option_as_existing_absolute_path(alternates alternates)
     qt_ir_get_option_as_cmake_flag_option(branch "CHECKOUT_BRANCH" checkout_branch_option)
 
     # The prefix for the cmake-style 'dictionary' that will be used by various functions.