Disable parallelism within debian/rules

Disable parallelism within debian/rules

Since both install-arch and install-indep run ‘$(MAKE) install’,
running them both concurrently creates race conditions, for example
where both jobs truncate and append to the same file.  Some
particularly problematic files are git-instaweb and GIT-BUILD-OPTIONS,
which apparently get regenerated on every build.

Prevent these targets from running concurrently using a .NOTPARALLEL
directive.  This tells make to build one target from debian/rules at a
time, while still allowing any -j<num> options to be passed on to
child $(MAKE)s so most of the parallelism of the build is not lost.

Noticed while investigating frequent build failures in the Ubuntu PPA.

Reported-by: Anders Kaseorg <andersk@MIT.EDU>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>