travis: run valgrind only once

Instead of running valgrind on each job, half of which are in release
mode and don't have much usable information for valgrind, perform an
debug build as part of allowed_failures and run valgrind on that one,
which should speed up the feedback we get from the builds.

Fix typo

No files merged may result in bogus merge conflict error

Fix attribute lookup in index for bare repos

When using a bare repo with an index, libgit2 attempts to read
files from the index.  It caches those files based on the path
to the file, specifically the path to the directory that contains
the file.

If there is no working directory, we use `git_path_dirname_r` to
get the path to the containing directory.  However, for the
`.gitattributes` file in the root of the repository, this ends up
normalizing the containing path to `"."` instead of the empty
string and the lookup the `.gitattributes` data fails.

This adds a test of attribute lookups on bare repos and also
fixes the problem by simply rewriting `"."` to be `""`.

Add Rust to the language bindings list

When auto follow tags, FETCH_HEAD should list only newly followed tags

Passing 0 as the length of the paths to check to git_diff_index_to_workdir
results in all files being treated as conflicting, that is, all untracked or
modified files in the worktree is reported as conflicting

Added test case to illustrate bogus conflicts detected if no files were merged, and untracked files exist in the workdir.

signature: don't allow empty emails

A signature is made up of a non-empty name and a non-empty email so
let's validate that. This also brings us more in line with git, which
also rejects ident with an empty email.

Restrict which refs can be the default branch

ssh: store error message immediately after a failed agent call

When the call to the agent fails, we must retrieve the error message
just after the function call, as other calls may overwrite it.

As the agent authentication is the only one which has a teardown and
there does not seem to be a way to get the error message from a stored
error number, this tries to introduce some small changes to store the
error from the agent.

Clearing the error at the beginning of the loop lets us know whether the
agent has already set the libgit2 error message and we should skip it,
or if we should set it.

Clean up transport lookup

Teach repository to use relative paths for git symbolic links

Several CppCat warnings fixed

Teach git_repository_init_ext to use relative paths for the gitlink
to the work directory. This is used when creating a sub repository
where the sub repository resides in the parent repository's
.git directory.

.editorconfig added to repository

Skip SSH clone tests if libgit2 not built with GIT_SSH defined

When the fetch refspec does not include the remote's default branch, it
indicates an error in user expectations or programmer error. Error out
in that case.

This lets us get rid of the dummy refspec which can never work as its
zeroed out. In the cases where we did not find a default branch, we set
HEAD detached immediately, which lets us refactor the "normal" path,
removing `found_branch`.

If the remote does not advertise HEAD, then it is unborn and we cannot
checkout that branch. Handle it the same way as an empty repo.

When cloning, we may be asking for a particular branch or subset of
branches. Make sure we test for that.

Add tests for the case when there are no branches on the remote and when
HEAD is detached but has the id of a non-branch. In both of these cases,
we should return ENOTFOUND.

Fix warning

Instead of using ifdefs to run the tests, use them to set when we expect
to support a particular scheme and always have the tests in the code.

It does the same as git_remote_supported_url() but has a name which
implies we'd check the URL for correctness while we're simply looking at
the scheme and looking it up in our lists.

While here, fix up the tests so we check all the combination of what's
supported.