submodule: check index for path and prefix before adding submodule

odb: mempack: fix leaking objects when freeing mempacks

When a ODB mempack gets free'd, we take no measures at all to free its
contents, most notably the objects added to the database, resulting in a
memory leak. Call `git_mempack_reset` previous to freeing the ODB
structures themselves, which takes care of releasing all associated
data structures.

types: remove unused git_merge_result

`git_merge_result` is currently unused in the codebase and generates a blank page in the [documentation](https://libgit2.github.com/libgit2/#HEAD/type/git_merge_result).

submodule: check path and prefix before adding submodule


submodule: fix test errors

checkout: change default strategy to SAFE

As per #4200, our default is quite surprising to users that expect checkout to just "do the thing".

Add myself to git.git-authors

odb: fix writing to fake write streams

In commit 7ec7aa4a (odb: assert on logic errors when writing objects,
2018-02-01), the check for whether we are trying to overflowing the fake
stream buffer was changed from returning an error to raising an assert.
The conversion forgot though that the logic around `assert`s are
basically inverted. Previously, if the statement

    stream->written + len > steram->size

evaluated to true, we would return a `-1`. Now we are asserting that
this statement is true, and in case it is not we will raise an error. So
the conversion to the `assert` in fact changed the behaviour to the
complete opposite intention.

Fix the assert by inverting its condition again and add a regression
test.

Our mempack ODB backend has no test coverage at all right now. Add a
simple test suite to at least have some coverage of the most basic
operations on the ODB.

online tests: update auth for bitbucket test

Update the settings to use a specific read-only token for accessing our
test repositories in Bitbucket.

Refactor `gitno_extract_url_parts`

RFC 3986 says that hostnames can be percent encoded.  Percent decode
hostnames in our URLs.

Now that we can decode percent-encoded strings as part of `git_buf`s,
use that decoder in `gitno_extract_url_parts`.

Use `git_buf_decode_percent` so that we can avoid allocating a temporary
buffer.

Introduce a function to take a percent-encoded string (URI encoded,
described by RFC 1738) and decode it into a `git_buf`.

online::clone: skip creds fallback test

At present, we have three online tests against bitbucket: one which
specifies the credentials in the payload, one which specifies the
correct credentials in the URL and a final one that specifies the
incorrect credentials in the URL.  Bitbucket has begun responding to the
latter test with a 403, which causes us to fail.

Break these three tests into separate tests so that we can skip the
latter until this is resolved on Bitbucket's end or until we can change
the test to a different provider.

pathspec: improve git_pathspec_flag_t doc rendering

Index parsing fixes

config: specify how we match the regular expressions