Small documentation improvements

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.

When we read from the list which `limit_list()` gives us, we need to check that
the commit is still interesting, as it might have become uninteresting after it
was added to the list.