Squashed 'deps/libgit2/' changes from de0c455..64dc248

64dc248 Merge pull request #2598 from libgit2/cmn/stacked-ignore
7bb6399 Merge pull request #2676 from libgit2/cmn/threading
f890a84 Merge pull request #2682 from libgit2/cmn/fetch-tags-refspec
6069042 ignore: don't leak rules into higher directories
0798b01 ignore: add failing test for a file mentioning the parent
b4e5432 Merge pull request #2688 from libgit2/cmn/ignore-file-trailing-cr
5c54e21 ignore: consider files with a CR in their names
3f8d005 Merge pull request #2686 from ethomson/rename_notfound
cce27d8 git_remote_rename: propogate GIT_ENOTFOUND
6d20006 remote: check the relevance of the refspec when updating FETCH_HEAD
0f838d2 remote: add a failing test for checking the current branch's upstream
15bea02 docs: explicitly document the threading caveats
fe6b51a ssl: separate locking init from general init
e083657 ssl: clear the OpenSSL locking function

git-subtree-dir: deps/libgit2
git-subtree-split: 64dc248577940c7874939bb82b6a557bafe8c388
Merge commit 'fc2fe491967e9f69fdb318cb7fae5970197c9210' into jacquesg/update-libgit2
Changes: update