odb: freshen existing objects when writing

Since writing multiple objects may all already exist in a single
packfile, avoid freshening that packfile repeatedly in a tight loop.
Instead, only freshen pack files every 2 seconds.

When writing an object, we calculate its OID and see if it exists in the
object database.  If it does, we need to freshen the file that contains
it.

sysdir: don't assume an empty dir is uninitialized

Fix outdated comment

Enable https transport for custom TLS streams

Don't try to determine when sysdirs are uninitialized.  Instead, simply
initialize them all at `git_libgit2_init` time and never try to
reinitialize, except when consumers explicitly call `git_sysdir_set`.

Looking at the buffer length is especially problematic, since there may
no appropriate path for that value.  (For example, the Windows-specific
programdata directory has no value on non-Windows machines.)

Previously we would continually trying to re-lookup these values,
which could get racy if two different threads are each calling
`git_sysdir_get` and trying to lookup / clear the value simultaneously.

mwindow: init mwindow files in git_libgit2_init

git_diff_file: move `id_abbrev`

Move `id_abbrev` to a more reasonable place where it packs more nicely
(before anybody starts using it).

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

Style guide says // style comments should be avoided.

SSH transport seems to be supported now.

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.