Windows XP updates

refs: add git_reference_is_tag

push: small documentation fix

fix tests on FreeBSD

238b7614 introduced a test for posix behaviour, but on FreeBSD some
of the structs and constants used aren't defined in <arpa/inet.h>.
Include the appropriate headers to get the tests working again on
FreeBSD.

Skip UTF-8 BOM in binary detection

When a git_buf contains a UTF-8 BOM, the three bytes comprising
that BOM are treated as unprintable characters.  For a small git_buf,
the three BOM characters overwhelm the printable characters.  This
is problematic when trying to check out a small file as the CR/LF
filtering will not apply.

Quiet down some warnings

Fix p_inet_pton on windows

p_inet_pton on Windows should set errno properly for callers.
Rewrite p_inet_pton to handle error cases correctly and add
test cases to exercise this function.

Use time(2) to get the time

We didn't use the added precision in gettimeofday, so remove it.
This prevents us from having an unnecessary reimplementation on
win32.

Don't expose git_hash_ctx since it's internal

And doing so makes the mingw build choke.

odb: move hashing to the frontend for streaming

odb: Straighten oid prefix handling

index: report when it's locked

When implementing the ssh testing, the move to the script made it so
the first test suite's exit code was ignored. Check whether the main
tests fail and exit with an error in that case.

Report the index being locked with its own error code in order to be
able to differentiate, as a locked index is typically the result of a
crashed process or concurrent access, both of which often require user
intervention to fix.

Bring Valgrind back!

If none of the backends support direct writes and we must stream the
whole file, we already know what the object's id should be; so use the
stream's functions directly, bypassing the frontend's hashing and
overwriting of our existing id.

Clarify the role of each function and in particular mention that there
is no need for the backend or stream to worry about the object's id,
as it will be given when `finalize_write` is called.

The frontend is in charge of calculating the id of the objects. Thus
the backends should treat it as a read-only value. The positioning in
the function signature made it seem as though it was an output
parameter.

Make the id const and move it from the front to behind the subject
(backend or stream).

push: handle tag chains correctly

Add "git init"-like example

This adds a new API for creating a signature that uses the
config to look up "user.name" and "user.email".

Give credit to PHP for the p_readlink function in posix_w32.c

When dealing with a chain of tags, we need to enqueue each of them
individually, which means we can't use `git_tag_peel` as that jumps
over the intermediate tags.

Do the peeling manually so we can look at each object and take the
appropriate action.