gitk: show detached HEAD if --all is specified

If HEAD is detached, 'gitk --all' does not show it. This is inconvenient
for frontend program, and for example git log does show the detached HEAD.

gitk uses git rev-parse to find a list of branches to show.
Apparently, the command does not include detached HEAD to output if
--all argument is specified. This has been discussed in [1] and stated
as expected behavior. So rev-parse's parameters should be tuned in gitk.

[1] http://thread.gmane.org/gmane.comp.version-control.git/255996
Merge 'gitk_show_detached_head'