branch --list: print useful info whilst interactive rebasing a detached HEAD

branch --list: print useful info whilst interactive rebasing a detached HEAD

When rebasing interactively (rebase -i), "git branch --list" prints
a line indicating the current branch being rebased. This works well
when the interactive rebase is initiated when a local branch is
checked out.

This doesn't play well when the rebase is initiated on a detached
HEAD. When "git branch --list" tries to print information related
to the interactive rebase in this case it tries to print the name
of a branch using an uninitialized variable and thus tries to
print a "null pointer string". As a consequence, it does not provide
useful information while also inducing undefined behaviour.

So, print the point from which the rebase was started when interactive
rebasing a detached HEAD.

Signed-off-by: Kaartic Sivaraam <kaartic.sivaraam@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>