Merge branch 'ah/show-ref-usage-string'

Both "git show-ref -h" and "git show-ref --help" illustrated that the
"--exclude-existing" option makes the command read list of refs
from its standard input.  Change only the "show-ref -h" output to
have a pair of "<>" around the placeholder that designate an input
file, i.e. "git show-ref --exclude-existing < <ref-list>".

* ah/show-ref-usage-string:
  show-ref: place angle brackets around variables in usage string