Update nodegit requirement from ^0.25.0 to ^0.26.0

[dependabot-preview]

Updates the requirements on nodegit to permit the latest version.

Changelog

Sourced from nodegit's changelog.

v0.26.0 (2019-09-09)

Full Changelog

Summary of changes

• Bumped libssh2 to 1.9 for security patch
• Remote.prototype.upload and Remote.prototype.updateTips should be async now

Merged PRs into NodeGit

• [GitRemote upload and updateTips are async #1720](https://github-redirect.dependabot.com/nodegit/nodegit/pull/1720)
• [Update libssh2 to 1.9 #1719](https://github-redirect.dependabot.com/nodegit/nodegit/pull/1719)

v0.25.1 (2019-08-13)

Full Changelog

Summary of changes

Security patch for LibGit2:

• A carefully constructed commit object with a very large number
  of parents may lead to potential out-of-bounds writes or
  potential denial of service.

• The ProgramData configuration file is always read for compatibility
  with Git for Windows and Portable Git installations. The ProgramData
  location is not necessarily writable only by administrators, so we
  now ensure that the configuration file is owned by the administrator
  or the current user.

Additionally:

• Stash should run much faster now.

Merged PRs into LibGit2

• [Parallelize checkout_create_the_new for perf #4205](https://github-redirect.dependabot.com/libgit2/libgit2/pull/4205)
• stash: avoid recomputing tree when committing worktree
• Variadic macros
• Add sign capability to git_rebase_commit
• remote: remove unused block of code
• Adjust printf specifiers in examples code
• config: check if we are running in a sandboxed environment
• Fix example checkout to forbid rather than require --
• editorconfig: update to match our coding style
• Compare buffers in diff example
• Include ahead_behind in the test suite
• config: separate file and snapshot backends
• object: deprecate git_object__size for removal

v0.25.0 (2019-08-09)

... (truncated)

Commits

• 0228707 Enable builds on tags
• 3699ee9 Bump to v0.26.0
• b6e5fac In testing, retry npm install because of unsolved race condition
• d071af0 Merge pull request #1720 from implausible/fix/async-remote-functions
• 46a3456 Merge pull request #1719 from implausible/patch/libssh2
• 95e0518 GitRemote upload and updateTips are async
• 129492d Update libssh2 to 1.9
• f2fea6a Merge pull request #1717 from henkesn/rebase-inmemory
• ee79dcb Add test for in-memory rebase
• b495715 Fix rebase using in-memory index
• Additional commits viewable in compare view

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[dependabot-preview]

Superseded by #3.