In libgit2: Move an enum out of an int bitfield in the HTTP transport.

In the parser: Use int bitfields and change some variable sizes to
better fit thir use. Variables that count the size of the data chunk
can only ever be as large as off_t. Warning 4127 can be ignored, as
nobody takes it seriously anyway.

From Emeric: change some variable declarations to keep MSVC happy.

Signed-off-by: Carlos Martín Nieto <carlos@cmartin.tk>