branch.autosetupmerge: allow boolean values, or "all"

branch.autosetupmerge: allow boolean values, or "all"

Junio noticed that switching on autosetupmerge unilaterally started
cluttering the config for local branches.  That is not the original
intention of branch.autosetupmerge, which was meant purely for
convenience when branching off of remote branches, but that semantics
got lost somewhere.

If you still want that "new" behavior, you can switch
branch.autosetupmerge to the value "all".  Otherwise, it is interpreted
as a boolean, which triggers setting up defaults _only_ when branching
off of a remote branch, i.e. the originally intended behavior.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>