execute_process: Document and test ENCODING default

The default was originally AUTO in commit 2ed473b3b8 (execute_process:
Add ENCODING option for Windows child process output, 2016-11-23,
v3.8.0-rc1~232^2) but was changed back to NONE by commit 07c3380a6e
(execute_process: Restore no-decoding default behavior, 2017-02-13,
v3.8.0-rc2~26^2) for CMake 3.8.  Later it was accidentally changed back
to AUTO by commit b783e62533 (cmExecuteProcessCommand: Port to
cmArgumentParser, 2019-03-25, v3.15.0-rc1~270^2) in CMake 3.15 but the
documentation was not updated.  Document the current default, AUTO, and
add a test case to verify it.

Issue: #26262

Help/command/execute_process.rst

@@ -140,12 +140,15 @@ Options:
  ``NONE``
    Perform no decoding.  This assumes that the process output is encoded
    in the same way as CMake's internal encoding (UTF-8).
-   This is the default.
+
+   This was the default in CMake 3.14 and older.
 
  ``AUTO``
    Use the current active console's codepage or if that isn't
    available then use ANSI.
 
+   This is the default since CMake 3.15.
+
  ``ANSI``
    Use the ANSI codepage.
 

Tests/RunCMake/execute_process/EncodingDefault-stderr.txt

@@ -0,0 +1,2 @@
+Chinese
+注意

Tests/RunCMake/execute_process/EncodingDefault.cmake

@@ -0,0 +1,3 @@
+# No explicit ENCODING option; fall back to default.
+set(CODEPAGE 54936)
+include(${CMAKE_CURRENT_LIST_DIR}/Encoding.cmake)

Tests/RunCMake/execute_process/RunCMakeTest.cmake

@@ -9,6 +9,7 @@ run_cmake_command(MergeOutputVars ${CMAKE_COMMAND} -P ${RunCMake_SOURCE_DIR}/Mer
 
 run_cmake(EncodingMissing)
 if(TEST_ENCODING_EXE)
+  run_cmake_script(EncodingDefault -DTEST_ENCODING_EXE=${TEST_ENCODING_EXE})
   run_cmake_script(EncodingAUTO -DTEST_ENCODING_EXE=${TEST_ENCODING_EXE})
   run_cmake_script(EncodingUTF-8 -DTEST_ENCODING_EXE=${TEST_ENCODING_EXE})
   run_cmake_script(EncodingUTF8 -DTEST_ENCODING_EXE=${TEST_ENCODING_EXE})