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