git hardocodes these as objects which exist regardless of whether they
are in the odb and uses them in the shell interface as a way of
expressing the lack of a blob or tree for one side of e.g. a diff.

In the library we use each language's natural way of declaring a lack of
value which makes a workaround like this unnecessary. Since git uses it,
it does however mean each shell application would need to perform this
check themselves.

This makes it common work across a range of applications and an issue
with compatibility with git, which fits right into what the library aims
to provide.

Thus we introduce the hard-coded empty blob and tree in the odb
frontend. These hard-coded objects are checked for before going to the
backends, but after the cache check, which means the second time they're
used, they will be treated as normal cached objects instead of creating
new ones.

submodules: stale module entries

We cannot know from looking at .gitmodules whether a directory is a
submodule or not. We need the index or tree we are comparing against to
tell us. Otherwise we have to assume the entry in .gitmodules is stale
or otherwise invalid.

Thus we pass the index of the repository into the workdir iterator, even
if we do not want to compare against it. This follows what git does,
which even for `git diff <tree>`, it will consider staged submodules as
such.

We consider an entry in .gitmodules to mean that we have a submodule at
a particular path, even if HEAD^{tree} and the index do not contain any
reference to it.

We should ignore that submodule entry and simply consider that path to
be a regular directory.

checkout_index: handle other stages

ignore: don't leak rules into higher directores

Threading and crypto libraries

remote: check for the validity of the refspec when updating FETCH_HEAD

A rule "src" in src/.gitignore must only match subdirectories of
src/. The current code does not include this context in the match rule
and would thus consider this rule to match the top-level src/ directory
instead of the intended src/src/.

Keep track fo the context in which the rule was defined so we can
perform a prefix match.

When we mention "src" in src/.gitignore, we wrongly consider src/ itself
to be ignored.

ignore: consider files with a CR in their names

We currently consider CR to start the end of the line, but that means
that we miss cases with CR CR LF which can be used with git to match
files whose names have CR at the end of their names.

The fix from the patch comes from Russell's comment in the issue.

This fixes #2536.

Propogate GIT_ENOTFOUND from git_remote_rename

Make config reading continue after hitting a missing include file.

Before trying to rtransform using the given refspec to figure out what
the name of the upstream branch is on the remote, we must make sure that
the target of the refspec applies to the current branch's upstream.

Changed context_lines and interhunk_lines to uint32_t to match struct s_xdemitconf

remote: unify the creation code

netops: return GIT_ECERTIFICATE when it fails the basic tests

Fix segmentation fault observed on OpenBSD/sparc64

Make the Visual Studio compiler happy

A non-readable mapping of a file causes an access violation in
the pack tests. Always use PROT_READ to work around this.

* Error-handling is cleaned up to only let a file-not-found error
  through, not other sorts of errors.  And when a file-not-found
  error happens, we clean up the error.
* Test now checks that file-not-found introduces no error.  And
  other minor cleanups.

The create function with default refspec is the same as the one with a
custom refspec, but it has the default refspec, so we can create the one
on top of the other.

An anonymous remote wouldn't create remote-tracking branches, so testing
we don't create them for TAGS_ALL is nonsensical. Furthermore, the name
of the supposed remote-tracking branch was also not one which would have
been created had it had a name.

Give the remote a name and test that we only create the tags when we
pass TAGS_ALL and that we do create the remote-branch branch when given
TAGS_AUTO.

When we update FETCH_HEAD we check whether the remote is the current
branch's upstream remote. The code does not check whether the current
refspec is relevant for this reference but always tries to perform the
reverse transformation, which causes it to error out if the refspec
doesn't match the reference.

Thanks to Pierre-Olivier Latour for the reproduction recipe.

Since it's not necessarily obvious, mention the merging state and how to
clear it.

When we first ask OpenSSL to verify the certfiicate itself (rather
than the HTTPS specifics), we should also return
GIT_ECERTIFICATE. Otherwise, the caller would consider this as a failed
operation rather than a failed validation and not call the user's own
validation.

For example, if you have

[include]
path = foo

and foo didn't exist, git_config_open_ondisk() would just give up
on the rest of the file.  Now it ignores the unresolved include
without error and continues reading the rest of the file.

Talk about sharing objects and error messages; but the most important
part is about what to do with the cryptographic libraries, which sadly
have to become to responsibility of the application.

Extract the lock-setting functions into their own, as we cannot assume
that it's ok for us to set this unconditionally.

We're freeing the memory which holds the locks so we must make sure that
the locking function doesn't try to use it.

revert tests: always use core.autocrlf=false in tests

Fixed the most obvious Clang static analyzer warnings