name-rev: strip trailing ^0 in when --name-only

236157 (Teach git-describe how to run name-rev, 2007-05-21) introduced
`git name-rev --name-only`, with the intent of using it to implement
`git describe --contains`.  According to the message, one of the primary
objectives of --name-only was to make the output of name-rev match that
of describe.

  $ git describe --contains --all master
  master

  $ git describe --contains --all master~1
  master~1

  $ git describe --contains --all v1.8.3~1
  v1.8.3~1

  $ git describe --contains --all v1.8.3
  v1.8.3^0

The last invocation unnecessarily prints a trailing "^0" (--stdin does
not suffer from this defect).  Fix this.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
name-rev doc: rewrite --stdin paragraph

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>