bsd.sites.mk: Support tag names with USE_GITLAB

Since quite some time, it's possible to download generated tarballs from
gitlab for a tag name without knowing the corresponding git hash. This
is preferable because you have a single source of truth about the
software version.

Support this by replacing GL_COMMIT with GL_TAGNAME which also accepts a
commit hash, similar to USE_GITHUB. Unfortunately, there's a catch, the
DISTFILE must be named exactly: ${GL_PROJECT}-${GL_TAGNAME}.tar.bz2,
otherwise it extracts in a directory that contains the commit hash. So,
we can't add the gitlab account or the revision suffix to make it work.

Therefore, detect whether GL_TAGNAME contains a commit hash, and if it
does, use the old behavior of deriving DISTNAME and DISTFILES.

Document in CHANGES, also add a DEV_WARNING similar to USE_GITHUB when
there's an URL in MASTER_SITES that looks like a gitlab URL. With
support for tags, there shouldn't be any reason left to hardcode such an
URL in MASTER_SITES.

PR: 248967
Approved by: portmgr (tcberner, mentor)
Differential Revision: https://reviews.freebsd.org/D37077