Support fetching/checkouts of non-branch/-tag git refs

[marxarelli]

Provides a source.git.ref field for specifying a specific non-branch
non-tag git ref at which to perform a checkout. Providing one will
result in a shallow (depth 1) clone of the repo and shallow subsequent
fetches.

The motivation for this change is to support sourcing of trigger
templates from Gerrit patchsets. Gerrit uses non-branch/-tag refs
extensively and specifically in this case stores change patchsets in
refs/changes/[slice]/[change]/[patchset]. While arbitrary refs are
kind of an obscure git feature, there very well may be other git based
systems utilizing them.

[marxarelli]

While I was implementing this change, I was a little confused about these fetchRefSpecs in store/git.go:

var (
	fetchRefSpec = []config.RefSpec{
		"refs/*:refs/*",
		"HEAD:refs/heads/HEAD",
	}
)

Why such an aggressive fetch of refs/*? Doesn't seem necessary if one is wanting a branch/tag (the default fetch git sets up for a remote should cover those) and could potentially pull down lots of extra data. (We have a repo where +100M unnecessary data would be pulled down from non-default refs.)

The second one seems to confuse git by creating an actual branch named HEAD which conflicts with the symbolic ref of the same name. If I connect to the sensor pod and do a git reflog on the cloned repo I get something like "ambiguous reference HEAD." I can create an issue with more info if that helps.

Anyway, since I don't know the history behind those refspecs, I didn't want to touch it, and simply included a conditional exemption for when source.git.ref is supplied. In that case, it really shouldn't be necessary to have anything but the specific ref.

[VaibhavPage]

left a comment, everything else looks good.

[marxarelli]

Thanks for the merge!

[mgoodness]

How can I use this? Built from master, pushed & deployed new images, but source.git.ref isn't recognized as a valid field.

Diff when applying my Sensor via Argo CD:

It reverts (drops the field) every time I sync.