t4080: do not let an absent python interpreter abort the test

The PYTHON prereq introduced in dcb7920 ("diff: add long-running
diff process via diff.<driver>.process") is set whenever Git was
built without `NO_PYTHON`, which says nothing about whether a
`python3` or `python` binary is actually on `PATH` at test time.
On Bash 5+ the test suite runs under implicit `set -e`, so when both
`command -v` lookups fail the resulting non-zero exit kills the
entire script before the first `test_expect_success` even runs;
prove then reports the test as `Tests: 0 Failed: 0` with no useful
TAP output and the job ends in `FATAL: Unexpected exit with code 1`.

This is exactly what happens on Git for Windows' `win test (9)`
shard: Git for Windows is built with `NO_PYTHON` unset (the Perl
component drags in the Python bindings, so the conditional in
`Makefile` evaluates to keep Python support enabled), but the
runtime `PATH` only carries `python` under MSYS2 in some shards and
neither in others. The same hazard exists on any environment that
builds with Python support but does not install a Python
interpreter alongside.

Add a third fallback that explicitly clears `PYTHON_PATH`. The
assignment chain then always exits 0, so `set -e` is happy, and the
later tests that actually need Python continue to be guarded by the
`PYTHON` prereq (which they already use via `test_expect_success
PYTHON ...`). When `PYTHON_PATH` is empty, those tests still skip
cleanly via the prereq check.

This fix targets the in-flight upstream topic; once that topic and
this fixup are both in upstream, the next merging-rebase will
collapse this commit naturally.

Assisted-by: Opus 4.7
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

t/t4080-diff-process.sh

@@ -6,7 +6,9 @@ test_description='diff process via long-running process'
 
 if test_have_prereq PYTHON
 then
-	PYTHON_PATH=$(command -v python3) || PYTHON_PATH=$(command -v python)
+	PYTHON_PATH=$(command -v python3) ||
+	PYTHON_PATH=$(command -v python) ||
+	PYTHON_PATH=
 fi
 
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