max_tree_depth: lower it for clangarm64 on Windows

Just as in b64d78a (max_tree_depth: lower it for MSVC to avoid
stack overflows, 2023-11-01), I encountered the same problem with the
clang builds on Windows/ARM64.

The symptom is an exit code 127 when t6700 tries to verify that `git
archive big` fails.

This exit code is reserved on Unix/Linux to mean "command not found".
Unfortunately in this case, it is the fall-back chosen by
Cygwin's `pinfo::status_exit()` method when encountering
the NSTATUS `STATUS_STACK_OVERFLOW`, see
https://github.com/cygwin/cygwin/blob/cygwin-3.6.1/winsup/cygwin/pinfo.cc#L171

I verified manually that the stack overflow always happens somewhere
around tree depth 1403, therefore 1280 should be a safe bound in these
instances.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho