--exclude-existing[=<pattern>]::
Make 'git show-ref' act as a filter that reads refs from stdin of the
- form "`{caret}(?:<anything>\s)?<refname>(?:{backslash}{caret}{})?$`"
+ form "`^(?:<anything>\s)?<refname>(?:\^{})?$`"
and performs the following actions on each:
(1) strip "{caret}{}" at the end of line if any;
(2) ignore if pattern is provided and does not head-match refname;