shell-common: centralize the scons and make aliases

Also, add a function to abstract out counting the number of cpus.
shell-common: add aliases for ninja, cninja, and gmake
git-fetch-all: search in the filtered entries not the full listing

Otherwise, we can end up fetching a repo twice.
git-fetch-all: provide more useful information when things fail
user/jszakmeister: make cn be nice