Introduce git_merge_head_id

Don't reset need_pack

While looping over multiple heads, an up-to-date head will clobber the `remote->need_pack` setting, preventing the rest of the machinery from building and downloading a pack-file, breaking fetches.

Update clar to 4b75388

RFC: Sandbox HOME during test execution

Fix use-after-free in submodule reload code and other memory leaks

If the first call to release a no-longer-existent submodule freed
the object, the check if a second is needed would dereference the
data that was just freed.

Fix segfault if gitmodules is invalid

When a submodule was inserted with a different path and name, the
return value from khash greater than zero was allowed to propagate
back out to the caller when it should really be zeroed.  This led
to a possible crash when reloading submodules if that was the
first time that submodule data was loaded.

The reload_all call could end up dereferencing a NULL pointer if
there was an error while attempting to load the submodules config
data (i.e. invalid content in the gitmodules file).  This fixes it.

Cleanup some warnings and mem leaks

Make submodules externally refcounted

Update behavior for untracked contained repositories

clone: don't overwrite original error message

This cleans up some places I missed that could hold onto submodule
references and cleans up the way in which the repository cache is
both reloaded and released so that existing submodule references
aren't destroyed inappropriately.

When a directory containing a .git directory (or even just a plain
gitlink) was found, libgit2 was going out of its way to treat it
specially.  This seemed like it was necessary because the diff
code was not originally emulating Git's behavior for untracked
directories correctly (i.e. scanning for ignored vs untracked items
inside).  Now that libgit2 diff mimics Git's untracked directory
behavior, the special handling for contained Git repos is actually
incorrect and this commit rips it out.

Callback function to hide commit and its parents in revision walker

`git_submodule` objects were already refcounted internally in case
the submodule name was different from the path at which it was
stored.  This makes that refcounting externally used as well, so
`git_submodule_lookup` and `git_submodule_add_setup` return an
object that requires a `git_submodule_free` when done.

env test needs to deref git_buf's ptr

Refactor the `git_merge` API

settings: use git_buf for returning strings

The default cache size is 256 megabytes, not megabits as claimed in the
docs.