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`.

`xlocale.h` only defines `regcomp_l` if `regex.h` was included as well.

Also change the test cases to actually test `p_regcomp` works with
a multibyte locale.

Improve revision walk preparation logic

When parsing tree entries from raw object data, we do not verify
that the tree entry actually has a filename as well as a valid
object ID. Fix this by asserting that the filename length is
non-zero as well as asserting that there are at least
`GIT_OID_RAWSZ` bytes left when parsing the OID.