Add GIT_BRANCH_ALL to git_branch_t enum

git_branch_t is an enum so requesting GIT_BRANCH_LOCAL | GIT_BRANCH_REMOTE is not possible as it is not a member of the enum (at least VS2013 C++ complains about it).

This fixes a regression introduced in commit a667ca8298193b3103c1dbdcb1f6c527e6e99eb2 (PR #1946).

Signed-off-by: Sven Strickroth <email@cs-ware.de>