git-describe: introduce --first-parent

so that git-describe searches first-parent history only when looking for
a named commit. This is useful for describing commits by tags on their
"main" (first-parent) branch; for example, on git.git:

git describe 22ffc39
v1.7.2.3-223-g22ffc39

git describe --first-parent 22ffc39
v1.7.2-104-g22ffc39

git describe --contains --first-parent is forbidden because git name-rev
(which is called by that) favors first-parent transversal already,
although not strictly so.
describe: document and test --first-parent