init-repository: Fix passing relative path to --alternates

When using --alternates with a relative path, nested submodules
(e.g., qtdeclarative/tests/auto/qml/ecmascripttests/test262) failed
because the alternates path wasn't adjusted for the changed working
directory depth. The --alternates option worked correctly with absolute
paths.

Fix by converting the passed alternates path to an absolute path.

Pick-to: 6.8 6.10 6.11
Fixes: QTBUG-142712
Change-Id: Ief7a87768d9ff17bb2832ae363f573ecae58d5ee
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

cmake/QtIRCommandLineHelpers.cmake

@@ -421,3 +421,17 @@ 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("${name}" path)
+    if(NOT EXISTS "${abspath}")
+        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()
+    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.