cmake: add dedicated error for -E compare_files invalid arguments

Return 2 when user provides invalid arguments

Fixes: #20815

Help/manual/cmake.1.rst

@@ -566,7 +566,8 @@ Available commands are:
 
 ``compare_files [--ignore-eol] <file1> <file2>``
   Check if ``<file1>`` is same as ``<file2>``. If files are the same,
-  then returns ``0``, if not it returns ``1``.  The ``--ignore-eol`` option
+  then returns ``0``, if not it returns ``1``.  In case of invalid
+  arguments, it retruns 2. The ``--ignore-eol`` option
   implies line-wise comparison and ignores LF/CRLF differences.
 
 ``copy <file>... <destination>``

Source/cmcmd.cxx

@@ -582,7 +582,7 @@ int cmcmd::ExecuteCMakeCommand(std::vector<std::string> const& args)
         filesDiffer = cmsys::SystemTools::TextFilesDiffer(args[3], args[4]);
       } else {
         ::CMakeCommandUsage(args[0].c_str());
-        return 1;
+        return 2;
       }
 
       if (filesDiffer) {

Tests/RunCMake/CommandLine/E_compare_files-invalid-arguments-result.txt

@@ -0,0 +1 @@
+2

Tests/RunCMake/CommandLine/E_compare_files-invalid-arguments-stderr.txt

@@ -0,0 +1 @@
+.*

Tests/RunCMake/CommandLine/RunCMakeTest.cmake

@@ -22,6 +22,7 @@ run_cmake_command(E_compare_files-different-eol ${CMAKE_COMMAND} -E compare_file
 run_cmake_command(E_compare_files-ignore-eol-same ${CMAKE_COMMAND} -E compare_files --ignore-eol ${RunCMake_SOURCE_DIR}/compare_files/lf ${RunCMake_SOURCE_DIR}/compare_files/crlf)
 run_cmake_command(E_compare_files-ignore-eol-empty ${CMAKE_COMMAND} -E compare_files --ignore-eol ${RunCMake_SOURCE_DIR}/compare_files/empty1 ${RunCMake_SOURCE_DIR}/compare_files/empty2)
 run_cmake_command(E_compare_files-ignore-eol-nonexistent ${CMAKE_COMMAND} -E compare_files --ignore-eol nonexistent_a nonexistent_b)
+run_cmake_command(E_compare_files-invalid-arguments ${CMAKE_COMMAND} -E compare_files file1.txt file2.txt file3.txt)
 run_cmake_command(E_echo_append ${CMAKE_COMMAND} -E echo_append)
 run_cmake_command(E_rename-no-arg ${CMAKE_COMMAND} -E rename)
 run_cmake_command(E_server-arg ${CMAKE_COMMAND} -E server --extra-arg)