The symlink destination is always concatenated to the original path. Fix
this by using `git_buf_sets` instead of `git_buf_puts`.

Diff fixes

merge_driver: fix const-correctness for source getters

submodule: catch when submodule is not staged on update

winhttp: disambiguate error messages when sending requests

When calling `git_submodule_update` on a submodule, we have to retrieve
the ID of the submodule entry in the index. If the function is called on
a submodule which is only partly initialized, the submodule entry may
not be added to the index yet. This leads to an assert when trying to
look up the blob later on.

Fix the issue by checking if the index actually holds the submodule's
ID and erroring out if it does not.

The function `diff_parsed_alloc` allocates and initializes a
`git_diff_parsed` structure. This structure also contains diff options.
While we initialize its flags, we fail to do a real initialization of
its values. This bites us when we want to actually use the generated
diff as we do not se the option's version field, which is required to
operate correctly.

Fix the issue by executing `git_diff_init_options` on the embedded
struct.

In a diff, the shortest possible hunk with a modification (that is, no
deletion) results from a file with only one line with a single character
which is removed. Thus the following hunk

    @@ -1 +1 @@
    -a
    +

is the shortest valid hunk modifying a line. The function parsing the
hunk body though assumes that there must always be at least 4 bytes
present to make up a valid hunk, which is obviously wrong in this case.
The absolute minimum number of bytes required for a modification is
actually 2 bytes, that is the "+" and the following newline. Note: if
there is no trailing newline, the assumption will not be offended as the
diff will have a line "\ No trailing newline" at its end.

This patch fixes the issue by lowering the amount of bytes required.

Now that the `git_diff_foreach` function does not depend on internals of
the `git_patch_generated` structure anymore, we can easily move it to
the actual diff code.

The current logic of `git_diff_foreach` makes the assumption that all
diffs passed in are actually derived from generated diffs. With these
assumptions we try to derive the actual diff by inspecting either the
working directory files or blobs of a repository. This obviously cannot
work for diffs parsed from a file, where we do not necessarily have a
repository at hand.

Since the introduced split of parsed and generated patches, there are
multiple functions which help us to handle patches generically, being
indifferent from where they stem from. Use these functions and remove
the old logic specific to generated patches. This allows re-using the
same code for invoking the callbacks on the deltas.

Under the existing logic, we try to load patch contents differently,
depending on whether the patch files stem from the working directory or
not. But actually, the executed code paths are completely equal to each
other -- so we were always the code despite the condition.

Remove the condition altogether and conflate both code paths.

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