Leftover OID -> ID changes

Buff up returning strings

MSVC doesn't like modern code neither

Make blame example compile on MSVC

A lot of the tests were checking for overflow, which we don't have
anymore, so we can remove them.

Again, we already did this internally, so simply remove the conversions.

Internally we already did everything with git_bufs, so this is just
exposing those functions with public names.

Since we now export that type, we can avoid making the user guess a
size.

Fix git_remote_dup & add missing const

Fix write_object.

Fix local push to file:// URL.

Following the rest of the series, use 'id' when refering to the value.

Same as the other commits in the series, we use 'id' when talking about
thing rather than the datatype.

In the same vein as the previous commits in this series.

Rename git_tree_entry_byoid() to _byid() as per the convention.

This was not converted when we converted the rest, so do it now.

This was left over when we did the general switch.

Drop parsing pack filename SHA1 part

Sometimes a zero byte file is just a zero byte file

Don't go to the ODB to resolve zero byte files in the workdir

Summarize empty messages

Preserve tree filemode in index during checkout

Don't try to determine whether the system supports file modes
when putting the tree data in the index during checkout.  The tree's
mode is canonical and did not come from stat(2) in the first place.

Expose patch serialization to git_buf

It's hard or even impossible to correctly free the string buffer
allocated by git_patch_to_str in some circumstances. Drop the function
so people have to use git_patch_to_buf instead - git_buf has a dedicated
destructor.

Returning library-allocated strings from libgit2 works fine on Linux,
but may cause problems on Windows because there is no one C Runtime that
everything links against. With libgit2 not exposing its own allocator,
freeing the string is a gamble.

git_patch_to_str already serializes to a buffer, then returns the
underlying memory. Expose the functionality directly, so callers can use
the git_buf_free function to free the memory later.

Merge improvements