Git plugin should (better) support downloading specific tags or commit hashes

[rohlem]

From what I can see, the Fetch-Tool plugin for Git features a branch option, but no dedicated way to specify a single tag or commit hash.
Putting a tag name into the branch field actually works for cloning, but on the next execution it checks if git branch has the tag name - which it doesn't, so it always forces re-cloning the repository.

Since the feature of specifying a commit hash is also missing, I would suggest adding an option for this (named commit_hash, or maybe just commit), and re-using this option for tag support.
The check on re-execution could work the same in all cases - compare the current git log hash with the hash from git log (options['commit_hash']).

As specifying both a branch and a commit seems incompatible to me, those options should probably be mutually exclusive.
(The plugin could verify that the commit is a parent of the given branch's current head, but I don't see the use case for that.)

If that sounds good, and I haven't just overlooked the relevant option somehow, I'd try implementing this tomorrow / over the course of the week.