debian/rules: set NUMJOBS to requested number of jobs, not ∞

debian/rules: set NUMJOBS to requested number of jobs, not ∞

debian/rules currently sets MAKEFLAGS using the following rule:

  NUMJOBS = $(patsubst parallel=%,%,$(filter PARALLEL=%,$(DEB_BUILD_OPTIONS)))
  MAKEFLAGS += -j$(NUMJOBS)

That will take any "PARALLEL=<num>" word (in all caps) from
DEB_BUILD_OPTIONS and then search for a "parallel=<num>" word
from which to take the <num> value and pass it to -j.  That is,
when this code trips, the effect is the same as the following:

  MAKEFLAGS += -j

That means to keep on spawning additional processes to build git
in parallel without limit.  The intent was to look for
"parallel=<num>" options (in lowercase) instead.  Do that.

Noticed on Debian's autobuilders.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>