Manage dependencies with git subtrees

deps-update.sh

#!/bin/sh -eu

deps () {
	# format is "key path repo branch"
	cat <<-"EOF"
	dep1	libs/dep1	git@github.com:User/lib1.git	master
	EOF
}

SEL=${1:-}
BR=${2:-}

deps | while read key path repo branch ; do
	test -n "$SEL" && test "$SEL" != "$key" && continue
	test -n "$BR" && branch=$BR
	git subtree pull --squash --prefix=$path $repo $branch
done

It can be called in tree different ways:

1. without parameters at all to pull updates to every dependency
   (hard-coded in this script) to their latest versions or branch specified in deps function;
2. with a single parameter to pull updates to given dependency (argument
   is short "name" of dependency hard-coded in this script) from its remote
   branch hard-coded in deps function;
3. with two arguments: first being the short "name" of dependency as in
   in previous case, and the second being specific branch name.