Fix last references to deprecated git_buf_free

revwalk: avoid walking the entire history when output is unsorted

Signed-off-by: Sven Strickroth <email@cs-ware.de>

Add mailmap support.

tree: remove unused functions

Link `mbedTLS` libraries in when `SHA1_BACKEND` == "mbedTLS"

editorconfig: allow trailing whitespace in markdown

Also matches git.

This matches git.

Not that it matters much anyway but...

docs: fix statement about tab width

diff: fix enum value being out of allowed range

The libgit2 project mostly follows the coding style of git and thus
the linux project. While those two projects use a recommended tab width
of eight spaces, we instruct users to set their editor's tab width to
four spaces. Fix this to say eight instead.