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

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.

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.