Special-casing null OIDs

README.md: add notes on how to report security issues

The null OID (hash with all zeroes) indicates a missing object in
upstream git and is thus not a valid object ID. Add defensive
measurements to avoid writing such a hash to the object database in the
very unlikely case where some data results in the null OID. Furthermore,
add shortcuts when reading the null OID from the ODB to avoid ever
returning an object when a faulty repository may contain the null OID.

In commit a96d3cc3f (cache-tree: reject entries with null sha1,
2017-04-21), the git.git project has changed its stance on null OIDs in
tree objects. Previously, null OIDs were accepted in tree entries to
help tools repair broken history. This resulted in some problems though
in that many code paths mistakenly passed null OIDs to be added to a
tree, which was not properly detected.

Align our own code base according to the upstream change and reject
writing tree entries early when the OID is all-zero.

odb: export mempack backend

Fixes #4492, #4496.

branch: refuse creating branches named 'HEAD'

refs: include " sorted " in our packed-refs header

message: update docs for git_message_prettify

tests: online::clone: fix memory leak due to not freeing URL

Since a625b092c (branch: correctly reject refs/heads/{-dash,HEAD},
2017-11-14), which is included in v2.16.0, upstream git refuses to
create branches which are named HEAD to avoid ambiguity with the
symbolic HEAD reference. Adjust our own code to match that behaviour and
reject creating branches names HEAD.

We used to hard-code the octothorpe as the comment character and the
documentation still mentions this even though we accept the comment character as
a parameter.

Update the line to indicate this and clean up the first paragraph a bit.

Implement message trailer parsing API

Memory leaks

This lets git know that we do in fact have written our packed-refs file
sorted (which is apparently not necessarily the case) and it can then use the
new-ish mmaped access which lets it avoid significant amounts of effort parsing
potentially large files to get to a single piece of data.

The test refs::iterator::foreach_name iterates through every reference
and copies its name into a local vector. While the test makes sure to
free the vector afterwards, the copied reference names are not being
free'd. Fix that.

References passed to the callback function of `git_reference_foreach`
are expected to be owned by the callback. As such, they are never being
freed by `git_reference_foreach`, but will have to be freed by the
caller. This small detail is never mentioned in the function's
documentation, though, making it easy to get wrong. Document this to
make it discoverable.

Upon downloading the pack file, the local transport will iterate through
every reference using `git_reference_foreach`. The function is a bit
tricky though in that it requires the passed callback to free the
references, which does not currently happen.

Fix the memory leak by freeing all passed references in the callback.

Two tests in network::fetchlocal explicitly set a cleanup function to
free and remove the created sandbox repositories. This is not necessary,
though, as the cleanup function executed after each test already takes
care of cleaning up after them. Remove the code to avoid needless code
duplication.

travis: fetch trusty dependencies from Bintray

The VM on Travis apparently will still proceed, but it's good practice.

The trusty dependencies are now hosted on Bintray.

cmake: use a FEATURE_SUMMARY call compatible with 3.0.2

When we print features, we make an effort to support all the way back to
pre-3.0. However, in the code for versions from 3 onward we call
`FEATURE_SUMMARY` with multiple kinds of elements to print in the same line.
This is only supported in CMake 3.1 and later, making the rather popular CMake
3.0.2 unable to build the library.

Use a single kind of element per invocation. This means we need to provide a
"description" text, which CMake provides for us if provide multiple kinds of
elements.

Make sure to include 'openssl' as a dep when building statically with SHA1DC

We might want OpenSSL to be the implementation for SHA-1 and/or TLS. If we only
want it for TLS (e.g. we're building with the collision-detecting SHA-1
implementation) then we did not indicate this to the systems including us a
static library.

Add OpenSSL to the list also during the TLS decision to make sure we say we
should link to it if we use it for TLS.

It is indeed a list of dependencies for those which include the static archive.
This is in preparation for adding two possible places where we might add openssl
as a dependency.

cmake: move the rule to find static archives close to building clar

If we're building static libraries, we want to use that for building our clar
binary. This is done in 49551254 (2017-09-22; cmake: use static dependencies
when building static libgit2) but that commit included the rule too early,
making it affect the search for iconv, meaning we did not find it when we were
building a static libgit2.

Move the rule to just before building clar, after we've included the rules for
building the library itself. This lets us find and link to the dynamic libiconv.

cmake: allow explicitly choosing SHA1 backend

Right now, if SHA1DC is disabled, the SHA1 backend is mostly chosen
based on which system libgit2 is being compiled on and which libraries
have been found. To give developers and distributions more choice,
enable them to request specific backends by passing in a
`-DSHA1_BACKEND=<BACKEND>` option instead. This completely replaces the
previous auto-selection.