Exercise the logic surrounding deinitialization of the libgit2
library as well as repeated concurrent de- and reinitialization.
This tries to catch races and makes sure that it is possible to
reinitialize libgit2 multiple times.

After deinitializing libgit2, we have to make sure to setup
options required for testing. Currently, this only includes
setting up the configuration search path again. Before, this has
been set up once in `tests/main.c`.

When threading is not enabled for libgit2, we keep global state
in a simple static variable. When libgit2 is shut down, we clean
up the global state by freeing the global state's dynamically
allocated memory. When libgit2 is built with threading, we
additionally free the thread-local storage and thus completely
remove the global state. In a non-threaded build, though, we
simply leave the global state as-is, which may result in an error
upon reinitializing libgit2.

Fix the issue by zeroing out the variable on a shutdown, thus
returning it to its initial state.

When trying to initialize and tear down global data structures
from different threads at once with `git_libgit2_init` and
`git_libgit2_shutdown`, we race around initializing data. While
we use `pthread_once` to assert that we only initilize data a
single time, we actually reset the `pthread_once_t` on the last
call to `git_libgit2_shutdown`. As resetting this variable is not
synchronized with other threads trying to access it, this is
actually racy when one thread tries to do a complete shutdown of
libgit2 while another thread tries to initialize it.

Fix the issue by creating a mutex which synchronizes `init_once`
and the library shutdown.

CI Improvements

Small documentation improvements

Our valgrind jobs haven't been failing for several builds by now.
This indicates that our tests are sufficiently stable when
running under valgrind. As such, any failures reported by
valgrind become interesting to us and shouldn't be ignored when
causing a build to fail.

Remove the valgrind job from the list of allowed failures.

When running a Coverity build, we have to provide an
authentication token in order to proof that we are actually
allowed to run analysis in the name of a certain project. As this
token should be secret, it is only set on the main repository, so
when we were requested to run the Coverity script on another
repository we do error out. But in fact we do also error out if
the Coverity analysis should _not_ be run if there is no
authentication token provided.

Fix the issue by only checking for the authentication token after
determining if analysis is indeed requested.

We used to only execute Coverity analysis on the 'development'
branch before commit 998f0016 (Refine build limitation,
2014-01-15), which refined Coverity build limitations. While we
do not really use the 'development' branch anymore, it does
still make sense to only analyze a single branch, as otherwise
Coverity might get confused.

Re-establish the restriction such that we only analyze libgit2's
'master' branch. Also fix the message announcing why we do not
actually analyze a certain build.

Trivial memory leak fixes in test suite

patch: minor documentation fix.

Fix @return description of git_patch_num_lines_in_hunk.

Squelch example warnings, enable CI

docs: GIT_OPT_ENABLE_STRICT_OBJECT_CREATION is enabled

Object parsing hardening

When parsing a commit, we will treat all bytes left after parsing
the headers as the commit message. When no bytes are left, we
leave the commit's message uninitialized. While uncommon to have
a commit without message, this is the right behavior as Git
unfortunately allows for empty commit messages.

Given that this scenario is so uncommon, most programs acting on
the commit message will never check if the message is actually
set, which may lead to errors. To work around the error and not
lay the burden of checking for empty commit messages to the
developer, initialize the commit message with an empty string
when no commit message is given.

We changed the defaults on strict object creation - it is enabled by
default.  Update the documentation to reflect that.

Fix the existence check for `regcomp_l`.