note: rename the id getter to git_note_id()

This was left over when we did the general switch.
index: rename an entry's id to 'id'

This was not converted when we converted the rest, so do it now.
commit: remvoe legacy 'oid' naming
tree: remove legacy 'oid' naming

Rename git_tree_entry_byoid() to _byid() as per the convention.
diff: rename the file's 'oid' to 'id'

In the same vein as the previous commits in this series.
diff: change id abbrev option's name to id_abbrev

Same as the other commits in the series, we use 'id' when talking about
thing rather than the datatype.
merge: rename _oid() -> id()

Following the rest of the series, use 'id' when refering to the value.
repository: move to use a git_buf for outputting strings

Since we now export that type, we can avoid making the user guess a
size.
branch: move to git_buf when outputting newly-allocated strings

Internally we already did everything with git_bufs, so this is just
exposing those functions with public names.
config: use git_buf for returning paths

Again, we already did this internally, so simply remove the conversions.
messsage: use git_buf in prettify()

A lot of the tests were checking for overflow, which we don't have
anymore, so we can remove them.
refspec: move to git_buf for outputting strings
MSVC doesn't like modern code neither
Make blame example compile on MSVC
Merge pull request #2079 from linquize/msvc-blame-example

Make blame example compile on MSVC
Merge pull request #2078 from linquize/msvc

MSVC doesn't like modern code neither
Merge pull request #2077 from libgit2/cmn/buf-out

Buff up returning strings
Merge pull request #2075 from libgit2/cmn/leftover-oid

Leftover OID -> ID changes