branch: description for non-existent branch errors

branch: description for non-existent branch errors

When the repository does not yet have commits, some errors describe that
there is no branch:

    $ git init -b first

    $ git branch --edit-description first
    error: No branch named 'first'.

    $ git branch --set-upstream-to=upstream
    fatal: branch 'first' does not exist

    $ git branch -c second
    error: refname refs/heads/first not found
    fatal: Branch copy failed

That "first" branch is unborn but to say it doesn't exists is confusing.

Options "-c" (copy) and "-m" (rename) show the same error when the
origin branch doesn't exists:

    $ git branch -c non-existent-branch second
    error: refname refs/heads/non-existent-branch not found
    fatal: Branch copy failed

    $ git branch -m non-existent-branch second
    error: refname refs/heads/non-existent-branch not found
    fatal: Branch rename failed

Note that "--edit-description" without an explicit argument is already
considering the _empty repository_ circumstance in its error.  Also note
that "-m" on the initial branch it is an allowed operation.

Make the error descriptions for those branch operations with unborn or
non-existent branches, more informative.

This is the result of the change:

    $ git init -b first

    $ git branch --edit-description first
    error: No commit on branch 'first' yet.

    $ git branch --set-upstream-to=upstream
    fatal: No commit on branch 'first' yet.

    $ git branch -c second
    fatal: No commit on branch 'first' yet.

    $ git branch [-c/-m] non-existent-branch second
    fatal: No branch named 'non-existent-branch'.

Signed-off-by: Rubén Justo <rjusto@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>