References for a repository are usually created inside of its gitdir.
When using worktrees, though, these references are not to be created
inside the worktree gitdir, but instead inside the gitdir of its parent
repository, which is the commondir. Like this, branches will still be
available after the worktree itself has been deleted.

The filesystem refdb currently still creates new references inside of
the gitdir. Fix this and have it create references in commondir.

The three link files "worktree/.git", ".git/worktrees/<name>/commondir"
and ".git/worktrees/<name>/gitdir" should always contain absolute and
resolved paths. Adjust the logic creating new worktrees to first use
`git_path_prettify_dir` before writing out these files, so that paths
are resolved first.

When creating a new worktree, we have to set up the initial data
structures. Next to others, this also includes the HEAD pseudo-ref.
We currently set it to the worktree respectively branch name, which is
actually not fully qualified.

Use the fully qualified branch name instead.

The working tree's parent path should not point to the parent's gitdir,
but to the parent's working directory. Pointing to the gitdir would not
make any sense, as the parent's working directory is actually equal to
both repository's common directory.

Fix the issue.

While we already provide functionality to look up a worktree from a
repository, we cannot do so the other way round. That is given a
repository, we want to look up its worktree if it actually exists.
Getting the worktree of a repository is useful when we want to get
certain meta information like the parent's location, getting the locked
status, etc.

Separate the logic of finding the worktree directory of a repository and
actually opening the working tree's directory. This is a preparatory
step for opening the worktree structure of a repository itself.

This will be used in later commits, where it becomes cumbersome to
always pass in a buffer.

winhttp: disambiguate error messages when sending requests

Skip uninteresting commits in revwalk timesort iterator

git_futils_readbuffer: don't compute sha-1

rebase: ignore untracked files in submodules

Fixes #4099

Don't compute the sha-1 in `git_futils_readbuffer_updated` unless the
checksum was requested.  This means that `git_futils_readbuffer` will
not calculate the checksum unnecessarily.

Support namespaced references again

These simple tests only ensure that we enforce the existence of a
namespace; these mirror the rugged tests, they are not exhaustive.

An untracked file in a submodule should not prevent a rebase from
starting.  Even if the submodule's SHA is changed, and that file would
conflict with a new tracked file, it's still OK to start the rebase
and discover the conflict later.

Signed-off-by: David Turner <dturner@twosigma.com>

git_commit_create: freshen tree objects in commit

Freshen the tree object that a commit points to during commit time.

Introduce (optional) SHA1 collision attack detection

We never set `SHA1_TYPE` to `builtin`.  Don't bother testing for it.

Include the SHA1 collision attack detection library from
https://github.com/cr-marcstevens/sha1collisiondetection

cmake: only enable supported compiler warning flags

We currently unconditionally enable the "-Wall" and "-Wextra" flags.
Some platforms rely on compilers which do not support these flags,
though. One of these platforms is Haiku, which does not support
"-Wextra" due to being stuck on GCC version 2.

Fix builds on such platforms by adding these flags only if supported by
the compiler.

tests: refs::create: fix memory leak

Fix: make reflog include "(merge)" for merge commits

This test ensures that the string '(merge)' is included in the reflog
when a merge commit is made.

This fixes issue #4094

tests: Add create__symbolic_with_arbitrary_content