gentree: combine spatches (unless using --gitdebug)

gentree: combine spatches (unless using --gitdebug)

Since spatch is rather slow, but can handle multiple spatches
concatenated in a single file, just do that and run it only
once rather than for each spatch. That shaves off some of the
runtime (startup etc. and finding affected files.)

On my system, I go from

real    9m42.616s
user    8m48.352s
sys     0m22.884s

to

real    9m1.948s
user    8m40.108s
sys     0m12.088s

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>