Undef lseek first
_InterlockedExchange expects a volatile LONG
git_pool_mallocsz takes an unsigned long
message: don't assume the comment char

The comment char is configurable and we need to provide a way for the
user to specify which comment char they chose for their message.
Merge pull request #2303 from jacquesg/mingw-lseek

WIP: Windows fixes
Merge pull request #2364 from libgit2/cmn/comment-char

message: don't assume the comment char