mingw: make exit_process() own the process handle on all paths

After "mingw: kill child processes in a gentler way", the ownership of
the HANDLE passed to `exit_process()` and `terminate_process_tree()` is
inconsistent. `terminate_process_tree()` always closes the handle;
`exit_process()` closes it on success and on the terminate-tree
fallback, but leaks it on the early return where GetExitCodeProcess()
fails or reports the process is no longer STILL_ACTIVE.

`mingw_kill()` compensated by closing the handle on its own error path,
which is a double-close on every error path that does not hit that one
leaky branch -- the callee has already closed the handle by then.
Coverity flagged the resulting use-after-free as CID 1437238.

Pin down the invariant that `exit_process()` and
`terminate_process_tree()` own the handle from the call onward and close
it on every return path; with that, the bogus close in `mingw_kill()`
goes away.

Assisted-by: Opus 4.7
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/mingw.c

@@ -2269,10 +2269,8 @@ int mingw_kill(pid_t pid, int sig)
 			}
 			ret = terminate_process_tree(h, 128 + sig);
 		}
-		if (ret) {
+		if (ret)
 			errno = err_win_to_posix(GetLastError());
-			CloseHandle(h);
-		}
 		return ret;
 	} else if (pid > 0 && sig == 0) {
 		HANDLE h = OpenProcess(PROCESS_QUERY_INFORMATION, FALSE, pid);

compat/win32/exit-process.h

@@ -159,6 +159,7 @@ static int exit_process(HANDLE process, int exit_code)
 		return terminate_process_tree(process, exit_code);
 	}
 
+	CloseHandle(process);
 	return 0;
 }