GCC 7 has introduced new warnings for implicit fallthrough in switch
statements. Whenever there is no branch in a case block, GCC will watch
out for some heuristics which indicate that the implicit fallthrough is
intended, like a "fallthrough" comment. The third-party http-parser code
manages to trick this heuristic in one case, even though there is a
"FALLTHROUGH" comment. Fortunately, GCC has also added a strictness
level to the -Wimplicit-fallthrough diagnostic, such that we can loosen
this heuristic and make it more lax.

Set -Wimplicit-fallthrough=1 in http-parser's CMake build instructions,
which is the strictest level that gets rid of the warning. This level
will treat any kind of comment as a "fallthrough" comment, which
silences the warning.

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.

The C89 standard states in §6.7.2.2 "Enumeration specifiers":

> The expression that defines the value of an enumeration constant shall
> be an integer constant expression that has a value representable as an
> int.

On most platforms, this effectively limits the range to a 32 bit signed
integer. The enum `git_diff_option_t` though recently gained an entry
`GIT_DIFF_INDENT_HEURISTIC = (1u << 31)`, which breaks this limit.

Fix the issue by using a gap in `git_diff_option_t`'s enum values. While
this has the benefit of retaining our API, it may break applications
which do not get recompiled after upgrading libgit2. But as we are
bumping the soversion on each release anyway and thus force a recompile
of dependents, this is not a problem.