t7502-commit: Skip SIGTERM test on Windows

t7502-commit: Skip SIGTERM test on Windows

The implementation of exec on Windows is just a rough approximation of the
POSIX behavior. In particular, no real process "overlay" happens (a new
process is spawned instead and the parent process waits until the child
terminates). In particular, the process ID cannot be taken by the exec'd
process. But there is one test in t7502-commit.sh that depends on this.
We have to skip it on Windows.

Signed-off-by: Johannes Sixt <redacted>