bisect reset: Allow resetting to any commit, not just a branch

bisect reset: Allow resetting to any commit, not just a branch

‘git bisect reset’ accepts an optional argument specifying a branch to
check out after cleaning up the bisection state.  This lets you
specify an arbitrary commit.

In particular, this provides a way to clean the bisection state
without moving HEAD: ‘git bisect reset HEAD’.  This may be useful if
you are not interested in the state before you began a bisect,
especially if checking out the old commit would be expensive and
invalidate most of your compiled tree.

Clarify the ‘git bisect reset’ documentation to explain this optional
argument, which was previously mentioned only in the usage message.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>