Search for tags with the *.*.* patten for version

This has been done in a way that I hope might be able to be upstreamed
in to versioneer. This is not garanteed but it would be good if it
could be.

buildstream/_version.py

@@ -43,6 +43,7 @@ def get_config():
     cfg.VCS = "git"
     cfg.style = "pep440"
     cfg.tag_prefix = ""
+    cfg.tag_regex = "*.*.*"
     cfg.parentdir_prefix = "BuildStream-"
     cfg.versionfile_source = "buildstream/_version.py"
     cfg.verbose = False
@@ -215,7 +216,7 @@ def git_versions_from_keywords(keywords, tag_prefix, verbose):
 
 
 @register_vcs_handler("git", "pieces_from_vcs")
-def git_pieces_from_vcs(tag_prefix, root, verbose, run_command=run_command):
+def git_pieces_from_vcs(tag_prefix, tag_regex, root, verbose, run_command=run_command):
     """Get version from 'git describe' in the root of the source tree.
 
     This only gets called if the git-archive 'subst' keywords were *not*
@@ -237,7 +238,7 @@ def git_pieces_from_vcs(tag_prefix, root, verbose, run_command=run_command):
     # if there isn't one, this yields HEX[-dirty] (no NUM)
     describe_out, rc = run_command(GITS, ["describe", "--tags", "--dirty",
                                           "--always", "--long",
-                                          "--match", "%s*" % tag_prefix],
+                                          "--match", "%s%s*" % (tag_prefix, tag_regex)],
                                    cwd=root)
     # --long was added in git-1.5.5
     if describe_out is None:
@@ -505,7 +506,7 @@ def get_versions():
                 "date": None}
 
     try:
-        pieces = git_pieces_from_vcs(cfg.tag_prefix, root, verbose)
+        pieces = git_pieces_from_vcs(cfg.tag_prefix, cfg.tag_regex, root, verbose)
         return render(pieces, cfg.style)
     except NotThisMethod:
         pass