Improve MSVC compiler, linker flags

Add git_push_options, to set packbuilder parallelism

Add FORCE_TEXT check into git_diff_blobs code path

Allow network operations to cancel

`git_diff_blobs` and `git_diff_blob_to_buffer` skip the step
where we check file attributes because they don't have a filename
associated with the data. Unfortunately, this meant they were also
skipping the check for the GIT_DIFF_FORCE_TEXT option and so you
could not force a diff of an apparent binary file.  This adds the
force text check into their code path.

Topic/remote isvalidname

Fix libgit2/libgit2sharp#318

reset: Allow the selective reset of pathspecs

Resolve a remote branch's remote

removed other references to api.html

Merge the push report into the refs to avoid a 3rd network call

diff: Add a callback to notify of diffed files

Fix Windows symlinks

As requested

The callback will be called for each file, just before the `git_delta_t` gets inserted into the diff list.

When the callback:
- returns < 0, the diff process will be aborted
- returns > 0, the delta will not be inserted into the diff list, but the diff process continues
- returns 0, the delta is inserted into the diff list, and the diff process continues

Instead of returning directly the pattern as the return value, I used an
out parameter, because the function also tests if the passed pathspecs
vector is empty. If yes, it considers that the path "matches", but in
that case there is no matched pattern per se.

No bitfields in public headers b/c packing is compiler-specific