When `GIT_ATTR_CHECK_INCLUDE_HEAD` is specified, read `gitattribute`
files that are checked into the repository at the HEAD revision.

Introduce `GIT_BLOB_FILTER_NO_SYSTEM_ATTRIBUTES`, which tells
`git_blob_filter` to ignore the system-wide attributes file, usually
`/etc/gitattributes`.

This simply passes the appropriate flag to the attribute loading code.

Allow system-wide attributes (the ones specified in
`/etc/gitattributes`) to be ignored if the flag
`GIT_FILTER_NO_SYSTEM_ATTRIBUTES` is specified.

Test that we can optionally ignore system attributes when filtering a
blob.

By default, `/etc/gitattributes` (or the system equivalent) is read to
provide attributes.  Ensure that, by default, this is read when
filtering blobs.

Users should now use `git_blob_filter`.

Move users of `git_blob_filtered_content` to `git_blob_filter`.

Provide a function to filter blobs that allows for more functionality
than the existing `git_blob_filtered_content` function.

Add a LF->CRLF conversion test to the blob filter.

fuzzer: use futils instead of fileops

w32: fix unlinking of directory symlinks

On most platforms it's fine to create symlinks to nonexisting files. Not
so on Windows, where the type of a symlink (file or directory) needs to
be set at creation time. So depending on whether the target file exists
or not, we may end up with different symlink types. This creates a
problem when performing checkouts, where we simply iterate over all blobs
that need to be updated without treating symlinks any special. If the
target file of the symlink is going to be checked out after the symlink
itself, then the symlink will be created as directory symlink and not as
file symlink.

Fix the issue by iterating over blobs twice: once to perform postponed
deletions and updates to non-symlink blobs, and once to perform updates
to symlink blobs.

When creating a symlink in Windows, one needs to tell Windows whether
the symlink should be a file or directory symlink. To determine which
flag to pass, we call `GetFileAttributesW` on the target file to see
whether it is a directory and then pass the flag accordingly. The
problem though is if create a symlink with a relative target path, then
we will check that relative path while not necessarily being inside of
the working directory where the symlink is to be created. Thus, getting
its attributes will either fail or return attributes of the wrong
target.

Fix this by resolving the target path relative to the directory in which
the symlink is to be created.

Add two more tests to verify that we're not deleting symlink targets,
but the symlinks themselves. Furthermore, convert several `cl_skip`s on
Win32 to conditional skips depending on whether the clar sandbox
supports symlinks or not. Windows is grown up now and may allow
unprivileged symlinks if the machine has been configured accordingly.

Several function calls to `p_stat` and `p_close` have no verification if
they actually succeeded. As these functions _may_ fail and as we also
want to make sure that we're not doing anything dumb, let's check them,
too.

When deleting a symlink on Windows, then the way to delete it depends on
whether it is a directory symlink or a file symlink. In the first case,
we need to use `DeleteFile`, in the second `RemoveDirectory`. Right now,
`p_unlink` will only ever try to use `DeleteFile`, though, and thus fail
to remove directory symlinks. This mismatches how unlink(3P) is expected
to behave, though, as it shall remove any symlink disregarding whether
it is a file or directory symlink.

In order to correctly unlink a symlink, we thus need to check what kind
of file this is. If we were to first query file attributes of every file
upon calling `p_unlink`, then this would penalize the common case
though. Instead, we can try to first delete the file with `DeleteFile`
and only if the error returned is `ERROR_ACCESS_DENIED` will we query
file attributes and determine whether it is a directory symlink to use
`RemoveDirectory` instead.

When initializing a repository, we need to check whether its working
directory supports symlinks to correctly set the initial value of the
"core.symlinks" config variable. The code to check the filesystem is
reusable in other parts of our codebase, like for example in our tests
to determine whether certain tests can be expected to succeed or not.

Extract the code into a new function `git_path_supports_symlinks` to
avoid duplicate implementations. Remove a duplicate implementation in
the repo test helper code.

Our file utils functions all have a "futils" prefix, e.g.
`git_futils_touch`. One would thus naturally guess that their
definitions and implementation would live in files "futils.h" and
"futils.c", respectively, but in fact they live in "fileops.h".

Rename the files to match expectations.

patch_parse: fix segfault due to line containing static contents

With commit dedf70ad (patch_parse: do not depend on parsed buffer's
lifetime, 2019-07-05), all lines of the patch are allocated with
`strdup` to make lifetime of the parsed patch independent of the buffer
that is currently being parsed. In patch b0893282 (patch_parse: ensure
valid patch output with EOFNL, 2019-07-11), we introduced another
code location where we add lines to the parsed patch. But as that one
was implemented via a separate pull request, it wasn't converted to use
`strdup`, as well. As a consequence, we generate a segfault when trying
to deallocate the potentially static buffer that's now in some of the
lines.

Use `git__strdup` to fix the issue.

ignore: fix determining whether a shorter pattern negates another

patch_parse: handle missing newline indicator in old file

patch_parse: do not depend on parsed buffer's lifetime

sha1: fix compilation of WinHTTP backend

repository: do not initialize HEAD if it's provided by templates

When using templates to initialize a git repository, then git-init(1)
will copy over all contents of the template directory. These will be
preferred over the default ones created by git-init(1). While we mostly
do the same, there is the exception of "HEAD". While we do copy over the
template's HEAD file, afterwards we'll immediately re-initialize its
contents with either the default "ref: refs/origin/master" or the init
option's `initial_head` field.

Let's fix the inconsistency with upstream git-init(1) by not overwriting
the template HEAD, but only if the user hasn't set `opts.initial_head`.
If the `initial_head` field has been supplied, we should use that
indifferent from whether the template contained a HEAD file or not. Add
tests to verify we correctly use the template directory's HEAD file and
that `initial_head` overrides the template.

Update `git_repository_init_ext` to use our typical style of error
handling. The function had multiple statements which didn't `goto out`
immediately but instead deferred it to later calls combined with `if`
statements.

The error handling in `git_repository_create_head` completely swallows
all error codes. While probably not too much of a problem, this also
violates our usual coding style.

Refactor the code to use a local `error` variable with the typical `goto
out` statements.

All tests in repo::template have a common pattern of first setting up
templates, then settung up the repository that makes use of those
templates via several init options. Refactor this pattern into two
functions `setup_templates` and `setup_repo` that handle most of that
logic to make it easier to spot what a test actually wants to check.

Furthermore, this also refactors how we clean up after the tests.
Previously, it was a combination of manually calling
`cl_fixture_cleanup` and `cl_set_cleanup`, which really is kind of hard
to read. This commit refactors this to instead provide the cleaning
parameters in the setup functions. All cleanups are then performed in
the suite's cleanup function.

The repo::template test suite makes use of quite a few local variables
that could be consolidated. Do so to make the code easier to read.

There's quite a lot of supporting code for our templates and they are an
obvious standalone feature. Thus, let's extract those tests into their
own suite to also make refactoring of them easier.

configuration: cvar -> configmap

Evict cache items more efficiently

clar: fix suite count

With the introduction of data variants for suites, we started
undercounting the number of suites as we didn't account for those that
were executed twice. This was then adjusted to count the number of
initializers instead, but this fails to account for suites without any
initializers at all.

Fix the suite count by counting either the number of initializers or, if
there is no initializer, count it as a single suite, only.

Ignore VS2017 specific files and folders

`cvar` is an unhelpful name.  Refactor its usage to `configmap` for more
clarity.