rebar_git_resource may produce incorrect versions

[max-au]

rebar supports {vsn, git} in myapp.app.src file, which is converted into actual version here:
https://github.com/erlang/rebar3/blob/master/src/rebar_git_resource.erl#L345-L355

However it is possible that git tag is not myapp-1.2.3. It is also possible that git tag contains directory separators. Hence https://github.com/erlang/rebar3/blob/master/src/rebar_git_resource.erl#L338 produces something exotic, for example:

063ecc4f (tag: myrepo/myapp/063eca9) Commit message 3
a3a268c1 (tag: myrepo/myapp/a3a26ae) Commit message 2
76543210 (tag: myrepo/myapp/1234567) Commit message 1

This will generate myapp.app file containing {vsn, "myrepo/myapp/063eca9"} - with slashes in application version. Then, relx will generate a release with sub-folders myrepo/myapp, and a single directory 063eca9 containing actual app.

There are at least 3 possible fixes to this:

1. If git tag does not form valid semver, replace it with 0.0.0 (current default behaviour if git fails to fetch tags).
2. If git tag contains slashes, strip everything but keep only the last portion. This will replace myrepo/myapp/063eca9 with just 063eca9
3. Replace characters that are not expected in vsn with dots - for example, transform "myrepo/myapp/063eca9" into "myrepo.myapp.063eca9

Out of these 3, last one looks to keep most information available. Are there any more options?

[tsloughter]

There may be a better git way of doing this as well now.

I don't remember why we even use that line instead of just git describe --tags --abbrev=0 which we fallback to on nomatch...

[max-au]

It there are no objections, I'd be happy to remove regex, and the whole call to
git -c color.ui=false log --oneline --no-walk --tags --decorate

It's rather slow and does not seem to help much. Also I can't find any documentation on how {vsn, git} is supposed to work.

Also, I found one example of even more exotic case: https://github.com/tomas-abrahamsson/gpb/blob/master/src/gpb.app.src#L24
Not sure if it's supported, but quite interesting case.

[ferd]

Yeah that's a supported format:

rebar3/src/rebar_utils.erl

Lines 742 to 743 in 8ff256d

	{cmd, CmdString} ->
	cmd_vsn_invoke(CmdString, rebar_app_info:dir(AppInfo));

however since it's an arbitrary command, we pick the string, trim it, and leave it at that

I'm fine more or less getting rid of the slow line (git -c color.ui=false log --oneline --no-walk --tags --decorate) and attached regexes, we'll be going through a major release candidate period for the next release so if there's a time to do it, it's as good as any.