Fix GIT_MERGE_OPTS_INIT on MSVC.
Added function-based initializers for every options struct.

The basic structure of each function is courtesy of arrbee.
Merge pull request #2014 from mgbowen/cpp-options-init

Function-based options initializers
Merge pull request #1997 from mgbowen/merge-options-init-fix

Fix GIT_MERGE_OPTS_INIT on MSVC.
Revert pull request #1997
Merge pull request #2165 from libgit2/revert-1997

Revert pull request #1997