tree.com uses the native UTF-16 encoding when writing to the console, just like cmd.exe and powershell.exe. So at first you'd expect redirecting the output to a file or pipe to also use Unicode. But tree.com, like most command-line utilities, encodes output to a pipe or disk file using a legacy codepage. (Speaking of legacy, the ".com" in the filename here is historical. In 64-bit Windows it's a regular 64-bit executable, not 16-bit DOS code.)
When writing to a pipe or disk file, some programs hard code the system ANSI codepage (e.g. 1252 in Western Europe) or OEM codepage (e.g. 850 in Western Europe), while some use the console's current output codepage (if attached to a console), which defaults to OEM. The latter would be great because you can change the console's output codepage to UTF-8 via chcp.com 65001. Unfortunately tree.com uses the OEM codepage, with no option to use anything else.
cmd.exe, on the other hand, at least provides a /u option to output its built-in commands as UTF-16. So, if you don't really need tree-formatted output, you could simply use cmd's dir command. For example:
cmd /u /c "dir /s /b" | clip
If you do need tree-formatted output, one workaround would be to read the output from tree.com directly from a console screen buffer, which can be done relatively easily for up to 9,999 lines. But that's not generally practical.
Otherwise PowerShell is probably your best option. For example, you could modify the Show-Tree script to output files in addition to directories.
