request much less data from git when inspecting a tag

request much less data from git when inspecting a tag

When calling "git ls-remote" or "git show-ref" they will output all existing
objects. Especially when working with stable kernel repositories this can be
a large amount of data. This can even cause pipe overflows when the internal
pipe buffer can not accept the >600kB of data that are returned in my case.

When checking a tag tell those commands to output only the matching refs, which
will greatly reduce the data returned and will speed up processing at both ends.

Signed-off-by: Rolf Eike Beer <eb@emlix.com>