CI: DMF: Generation of ChangeLog depends on presence of master branch in index

[jimklimov]

Common ChangeLog starting point is tag v2.6.0 which is available among other tags in CI builds. The branches however are pulled on a need-to-know basis, so PRs only have their source and target, e.g.

$ git branch -a
* (detached from 1a60b39)
  remotes/origin/FTY
  remotes/origin/PR-2544

DMF recipes were changed to show log differences from a master branch to current DMF/FTY codebase instead. However, in non-persistent workspaces the branch is not known:

$ git checkout master
error: pathspec 'master' did not match any file(s) known to git.

fatal: ambiguous argument 'master..HEAD': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'
  DOC-CHANGELOG-ASCIIDOC	SKIP: FAILED to resolve input or output filename with this make implementation, or input was not generated!

The solution is probably to also build DMF/FTY ChangeLog starting from a tag, whether fixed or e.g. latest NUT release now that we update the branch often, heading towards full merge per #1316.

Note that this is not a very pressing issue, since the majority of practically useful complete builds are done either from the master branch iterations, or with PR's aiming to merge into it, so the branch is usually known to CI agents.

[jimklimov]

Note: similar issue noted with work on gitlog2version.sh. It goes to some extremes to find the newest TRUNK among known git remotes that might have a variant of master history cached in the local index, if one to use was not specified explicitly.