The commit "Fix get_filename_component ABSOLUTE mode" broke REALPATH
treatment of relative paths because it stopped storing the absolute path
in local variable 'filename'. This commit fixes the call to GetRealPath
to use the proper local variable and adds a test.
This patch from Philip Lowman creates a REALPATH mode in the
get_filename_component command. It is like ABSOLUTE, but will also
resolve symlinks (which ABSOLUTE once did but was broken long ago).
See issue #8423.