branch: introduce --show-current display option

branch: introduce --show-current display option

When called with --show-current, git branch will print the current
branch name and terminate. Only the actual name gets printed,
without refs/heads. In detached HEAD state, nothing is output.

Intended both for scripting and interactive/informative use.
Unlike git branch --list, no filtering is needed to just get the
branch name.

Signed-off-by: Daniels Umanovskis <daniels@umanovskis.se>
Signed-off-by: Junio C Hamano <gitster@pobox.com>