When importing using a tag for the upstream point to use, use the given
tag provided instead of running describe against it again as this might
result in the wrong tag being returned.
The documentation for git describe states that when provided a tag,
`git describe <tag>` will simply return the given tag, however
when it is not an annotated tag it is possible for there to be multiple
tags on the same commit and `git describe` may return any of them
instead of the originally given tag. Explicitly add a check to see if
the commit(ish) reference to import from is a tag and skip calling `git
describe` in such a case.
Change-Id: I7c6c3c2eb138089ccbc13d773da3d8a4a8efe673
Related-Bug: #1625878
f243bd6355