[BuildStream] Merging 'git_tag' and 'git'

[Tom Mewett <tom mewett codethink co uk>]

Hi,

I would like to re-open the discussion around merging into the 'git'
source the functionality added by the 'git_tag' source in bst-external.
I started work on this in !1774.

[!1774] https://gitlab.com/BuildStream/buildstream/merge_requests/1774

The main additions to 'git_tag' are:

1.  It will fetch only the given ref when given an exact tag ref
    (*-0-g*)
2.  It can be configured to choose the latest tagged commit when
    tracking
3.  It can track multiple branches; it chooses the latest
    commit/tag found on any when tracking

As I see it, (1) and (3) are uncontroversial and could be added right
away. I welcome any comments on this.

(2) has been the subject of previous discussion, in #220. As far as I
can read, the problem blocking this being added is the one raised by
Sander. It seems a discussion took place in person, which concluded
with the decision that this is "functionality that belongs outside of
BuildStream."

[#220] https://gitlab.com/BuildStream/buildstream/issues/220

"It is BuildStream's responsibility to resolve a tracking branch to a
ref. It is the user's responsibility to specify the tracking branch to
resolve."

As I see it, nothing about a tag-tracking feature makes any of this
untrue. It is simply the user informing BuildStream *how* to resolve
the tracking branch to the ref.

Tag tracking is already employed heavily by many BuildStream projects I
have worked on, including freedesktop-sdk. It is a convenient way to
update elements in a project which builds released versions of
software. In my opinion, it is *absolutely* something that belongs in
BSt.

It would be great to hear anyone else's thoughts on this.

-- 
Tom Mewett <tom mewett codethink co uk>