Fix leaks, some warnings and an error

Coverity user model

remote: Handle missing config values when deleting a remote

And give it a default so that some compilers don't (unnecessarily)
complain.

After 1cd65991, we were passing a pointer to an `unsigned long` to
a function that now expected a pointer to a `size_t`.  These types
differ on 64-bit Windows, which means that we trash the stack.

Use `size_t`s in the packbuilder to avoid this.

Compare the preimage to the image; don't compare the preimage to itself.

ci: point to the homebrew libssh2 install

Somehow I ended up with the following in my ~/.gitconfig:
[branch "master"]
remote = origin
merge = master
rebase = true

I assume something went crazy while I was running the git.git tests
some time ago, and that I never noticed until now.

This is not a good configuration, but it shouldn't cause problems. But
it does. Specifically, if you have this in your config, and you
perform the following set of actions:

create a remote
fetch from that remote
create a branch off of the remote master branch called "master"
delete the branch
delete the remote

The remote delete fails with the message "Could not find key
'branch.master.rebase' to delete". This is because it's iterating over
the config entries (including the ones in the global config) and
believes that there is a master branch which must therefore have these
config keys.

https://github.com/libgit2/libgit2/issues/3856

Add get user agent functionality.

Fixed bug while parsing INT64_MIN

find_repo: Clean up and simplify logic

Add GIT_REPOSITORY_OPEN_FROM_ENV flag to respect $GIT_* environment vars

Document GIT_REPOSITORY_OPEN_NO_DOTGIT and GIT_REPOSITORY_OPEN_FROM_ENV.

Include conflicts in `git_index_read_index`

Ensure that we include conflicts when calling `git_index_read_index`,
which will remove conflicts in the index that do not exist in the new
target, and will add conflicts from the new target.

Most of `git_index_read_index` is common to reading any iterator.
Refactor it out in case we want to implement `read_tree` in terms of it
in the future.

submodule: Try to fetch when update fails to find the target commit.

blame: increment reference count for origin's commit

When we create a blame origin, we try to look up the blob that is
to be blamed at a certain revision. When this lookup fails, e.g.
because the file did not exist at that certain revision, we fail
to create the blame origin and return `NULL`. The blame origin
that we have just allocated is thereby free'd with
`origin_decref`.

The `origin_decref` function does not only decrement reference
counts for the blame origin, though, but also for its commit and
blob. When this is done in the error case, we will cause an
uneven reference count for these objects. This may result in
hard-to-debug failures at seemingly unrelated code paths, where
we try to access these objects when they in fact have already
been free'd.

Fix the issue by refactoring `make_origin` such that we only
allocate the object after the only function that may fail so that
we do not have to call `origin_decref` at all. Also fix the
`pass_blame` function, which indirectly calls `make_origin`, to
free the commit when `make_origin` failed.

Reading patch files

Improve the README

When showing copy information because we are duplicating contents,
for example, when performing a `diff --find-copies-harder -M100 -B100`,
then show copy from/to lines in a patch, and do not show context.
Ensure that we can also parse such patches.

Patches may have no hunks when there's no modifications (for example,
in a rename).  Handle them.

Test that we can create a diff file, then parse the results and
that the two are identical in-memory.

find_repo had a complex loop and heavily nested conditionals, making it
difficult to follow.  Simplify this as much as possible:

- Separate assignments from conditionals.
- Check the complex loop condition in the only place it can change.
- Break out of the loop on error, rather than going through the rest of
  the loop body first.
- Handle error cases by immediately breaking, rather than nesting
  conditionals.
- Free repo_link unconditionally on the way out of the function, rather
  than in multiple places.
- Add more comments on the remaining complex steps.

git_repository_open_ext provides parameters for the start path, whether
to search across filesystems, and what ceiling directories to stop at.
git commands have standard environment variables and defaults for each
of those, as well as various other parameters of the repository. To
avoid duplicate environment variable handling in users of libgit2, add a
GIT_REPOSITORY_OPEN_FROM_ENV flag, which makes git_repository_open_ext
automatically handle the appropriate environment variables. Commands
that intend to act just like those built into git itself can use this
flag to get the expected default behavior.

git_repository_open_ext with the GIT_REPOSITORY_OPEN_FROM_ENV flag
respects $GIT_DIR, $GIT_DISCOVERY_ACROSS_FILESYSTEM,
$GIT_CEILING_DIRECTORIES, $GIT_INDEX_FILE, $GIT_NAMESPACE,
$GIT_OBJECT_DIRECTORY, and $GIT_ALTERNATE_OBJECT_DIRECTORIES.  In the
future, when libgit2 gets worktree support, git_repository_open_env will
also respect $GIT_WORK_TREE and $GIT_COMMON_DIR; until then,
git_repository_open_ext with this flag will error out if either
$GIT_WORK_TREE or $GIT_COMMON_DIR is set.