Add the ability to build from any commit
I'd like to be able to build from any selected commit on the 5.1 branch. In that case, a KiCad source repo SHA can be specified in place of a tag, but the other repos, like i18n, doc, etc won't have that SHA, and the build will fail.
This patch adds a fallback to build from the HEAD of any repo that is missing a specified SHA.
For example, I can do:
./builder.sh -t e99a2181ffe83732c68e100f3a22441f42404c55 -m fedora-30-x86_64
This will use the e99a218... commit from the source repo, along with the HEADs of all other repos.
Also, update the help to mention f30 rather than f27.