Squashed 'deps/libgit2/' changes from 46a13f3..3a495c1

3a495c1 Merge pull request #2558 from libgit2/cmn/signature-empty-email
76e3c43 signature: don't allow empty emails
31e752b Merge pull request #2511 from libgit2/cmn/remote-default-restrict
1e71354 reflog: Error handling on empty reflog
0972443 Merge pull request #2553 from libgit2/cmn/ssh-errors
2dc399a ssh: store error message immediately after a failed agent call
89e05e2 Merge pull request #2543 from libgit2/cmn/known-transports
4c95804 Merge pull request #2530 from libgit2/jamill/relative_gitlink
eb939c5 Merge pull request #2552 from KindDragon/master
bc73762 Introduce option to use relative paths for repository work directory
0ee9f31 Introduce git_path_make_relative
6f3082d Merge pull request #2550 from KindDragon/patch-2
0ac9f9a Merge pull request #2545 from linquize/no-ssh
5cd81bb Several CppCat warnings fixed
098bb9d .editorconfig added to repository
15c30b7 clone: handle overly restrictive refspecs
e128a1a clone: correct handling of an unborn HEAD
538f908 remote: add test for single-branch clone
f2ffab6 remote: add tests for remote-branch edge cases
d4cb23e Merge pull request #2548 from linquize/warning5
bd465f9 Fix warning
fe3b678 Skip SSH clone tests if libgit2 not built with GIT_SSH defined
05ac705 remote: test for supported URLs in a single place
ba67c07 remote: get rid of git_remote_valid_url()
bd3854a transport: return ENOTFOUND for HTTPS and SSH when they're not supported
dbc7785 transport: distinguish between unknown and unsupported transports
69b5845 Merge pull request #2544 from linquize/test-push
aff7001 MSVC does not support zero size array
4625446 clone: support remotes with references but no branches
0cdaa37 remote: short-circuit the default branch check if there is none
94412b0 remote: assert what we want to happen when ther is no default branch
26bf3a5 travis: no need to clean out the test repository
3895260 remote: restrict default branch to branches namespace

git-subtree-dir: deps/libgit2
git-subtree-split: 3a495c19bd280f5455047a9ac0e936f2c2f2f9a9
Merge commit '626e5303c129bbb93c79f97b72822d46ad91865d' into jacquesg/update-libgit2
Remote.xs: Removed is_url_valid()
Changes: update
Merge branch 'jacquesg/update-libgit2'