The struct 'git_filelock' represents an atomically-locked
file, git-style.

Locked files can be modified atomically through the new file lock
interface:

int git_filelock_init(git_filelock *lock, const char *path);
int git_filelock_lock(git_filelock *lock, int append);
void git_filelock_unlock(git_filelock *lock);
int git_filelock_commit(git_filelock *lock);
int git_filelock_write(git_filelock *lock, const char *buffer, size_t length);

Signed-off-by: Vicent Marti <tanoku@gmail.com>