execute_process: Improve stdout/stderr merging

Use the KWSys Process "MergeOutput" option to give the child process
the same pipe (or file) for both stdout and stderr.  This allows
natural merging of stdout and stderr together instead of merging
on arbitrary buffered read boundaries as before.

Help/release/dev/execute_process-merge-output.rst

@@ -0,0 +1,5 @@
+execute_process-merge-output
+----------------------------
+
+* The :command:`execute_process` command learned to support specifying
+  the same file for ``OUTPUT_FILE`` and ``ERROR_FILE``.