C++ style comment ("//") are not specified by the ISO C90 standard and
thus do not conform to it. While libgit2 aims to conform to C90, we did
not enforce it until now, which is why quite a lot of these
non-conforming comments have snuck into our codebase. Do a tree-wide
conversion of all C++ style comments to the supported C style comments
to allow us enforcing strict C90 compliance in a later commit.

Rename git_merge_head to git_annotated_commit, as it becomes used
in more operations than just merge.

git_merge_analysis will now return GIT_MERGE_CONFIG_NO_FASTFORWARD
when merge.ff=false and GIT_MERGE_CONFIG_FASTFORWARD_ONLY when
merge.ff=true