Add vendored feature

[arxanas]

As per #721, some consumers may
prefer to build libgit2 themselves to simplify distribution.

To test the vendored version, you can run:

cargo build --features vendored-libgit2`

That being said, the test branch::tests::name_is_valid fails on macOS. This
test is currently broken as per
#721 (comment)

[alexcrichton]

Personally I think this is a case where the burden of getting things working shouldn't be placed on the contributor but rather this crate itself. If libgit2 is stable enough that there's different versions in the wild that we're using, then the API of this crate should be adapted when using an older version or the system version should be rejected if it isn't new enough.

[arxanas]

the burden of getting things working shouldn't be placed on the contributor

Who do you mean by the "contributor" in this case? The prospective contributor to git2-rs, or to libgit2? I don't follow how adding a vendored feature affects either of those parties.

the API of this crate should be adapted when using an older version or the system version should be rejected if it isn't new enough

Whose use-case is this helping? I guess you mean for people writing libraries on top of git2-rs, who would prefer to integrate with the system libgit2 if available?

In the case of end-user applications like my https://github.com/arxanas/git-branchless or @extrawurst's https://github.com/extrawurst/gitui, I think we would like to have an option where we can significantly simplify the distribution/compilation for our users who aren't consuming our applications via package manager, such as when compiling from source or downloading an executable directly from Github Releases.

[alexcrichton]

Sorry by contributor I actually mean the opposite, rather users of the crate (it's early here...). I'd prefer to not burden users with the choice of how to build the native library and improve the detection at build time.

Another option though would be to use an env var instead, I don't think features work out great for a build option like this because features are part of the crate graph and this is more of a build-time concern.

[kim]

Just as a data point: we needed to use a patched version of libgit2, but were bitten by the fact that git2 currently prefers the system libgit2 if it finds it. We ended up enabling the zlib-ng feature to force the vendored version.

So, as a user, I would’ve been less surprised by a feature named as proposed here.

[alexcrichton]

Ok let's just add this in then.