Move definition of enum branch_track from cache.h to branch.h

Move definition of enum branch_track from cache.h to branch.h

'branch_track' feels more closely related to branching, and it is
needed later in branch.h; rather than #include'ing cache.h in branch.h
for this small enum, just move the enum and the external declaration
for git_branch_track to branch.h.

Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>