Fix interaction between limited flag and sorting over resets

deps: fix implicit fallthrough warning in http-parser

Fix assorted leaks found via fuzzing

Fix type confusion in git_smart__connect

Verify ref_pkt's are long enough

git__free is insufficient if the packet is a git_pkt_ref or another
type that requires freeing referenced structures.

If the remote sends a too-short packet, we'll allow `len` to go
negative and eventually issue a malloc for <= 0 bytes on

```
pkt->head.name = git__malloc(alloclen);
```

Nothing verifies that t->refs[0] is a GIT_PKT_REF. A remote can send
another packet type, ultimately resulting in a type confusion in
`git_smart__detect_caps`

Currently we fail to clear the sorting flag for revwalks when resetting.
This caused a poor interaction with the limited flag during a recent
patch. This patch clears the revwalk sorting flag and causes it to no
longer persist over resets.

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.

Config parser cleanups

Buffers which ran out of memory will never have any memory attached to
them. As such, it is not necessary to call `git_buf_free` if the buffer
is out of memory.

The `parse_variable` function has two out parameters `var_name` and
`var_value`. Currently, those are not being sanitized to `NULL`. when.
any error happens inside of the `parse_variable` function. Fix that.
While at it, the coding style is improved to match our usual coding
practices more closely.

The function `git_config_parse` uses several callbacks to pass data
along to the caller as it parses the file. One design shortcoming here
is that strings passed to those callbacks are expected to be freed by
them, which is really confusing.

Fix the issue by changing memory ownership here. Instead of expecting
the `on_variable` callbacks to free memory for `git_config_parse`, just
do it inside of `git_config_parse`. While this obviously requires a bit
more memory allocation churn due to having to copy both name and value
at some places, this shouldn't be too much of a burden.

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