config: learn the "onbranch:" includeIf condition

config: learn the "onbranch:" includeIf condition

Currently, if a user wishes to have individual settings per branch, they
are required to manually keep track of the settings in their head and
manually set the options on the command-line or change the config at
each branch.

Teach config the "onbranch:" includeIf condition so that it can
conditionally include configuration files if the branch that is checked
out in the current worktree matches the pattern given.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>