LibGit2 refactor

The LibGit2 module could do with some work. Some possible changes

• functions should mirror CLI (command line interface) git functionality when possible
  • e.g. LibGit2.checkout instead of LibGit2.branch!
  • possible exceptions when CLI does multiple things in one go
• consistent use of ! suffix
• use Enum and FlagEnum (pending #19470) for constants whenever possible
• don't swallow errors:
  • e.g. rebase! will silently abort if it fails
  • revparse as well
• Normalize and check path inputs, ref #18724.
• Clean up a few objects which allocate memory in a confusing way:
  • StrArrayStruct: manually calls Libc.malloc, but then frees via LibGit2 git_strarray_free
  • Buffer: is an immutable object, but needs a finalizer
  • SignatureStruct, GitSignature, Signature objects should be simplified
• split out LibGit2.get into more usefully-named functions
  • Maybe via constructors? e.g. GitBlob(repo::GitRepo, hash::GitHash)
  • when returning a GitObject, automatically resolve its type via git_object_type function
  • LibGit2.get(T, repo, oid) should throw an error (TypeError?) if incorrect type T is used.
• Make GitConfig objects act like an Associative{String,String} (i.e. overload getindex/setindex!)
• rename GitAnyObject to GitUnknownObject #19935
• rename Oid to GitHash #19878
• Different types for full GitHash and short hashes: see https://github.com/JuliaLang/julia/pull/19878#issuecomment-270714556
• More sensible errors and return values
  • LibGit2.fetch should return nothing (since it will throw an error for any other value)
  • LibGit2.reset! and rebase should return current HEAD commit
  • LibGit2.upstream and LibGit2.lookup_branch should throw errors if not found, instead of returning nothing
  • Don't use "zero" GitHash objects as null values.
• Support newbase option for rebase! (cf https://github.com/JuliaLang/julia/pull/19651#discussion_r94463640)
• Use correct iteration protocol for Base.next(::LibGit2.Rebase) (or until new iteration protocol is in place)
• Combine LibGit2.owner and LibGit2.repository
• Get rid of a lot of the with and explicit finalize/close calls (pending #19660)
• Sensible show methods for various objects
  • GitBlob
  • GitCommit
  • GitRemote
  • GitSignature
  • GitTag
  • GitTreeEntry
  • GitReference
  • GitIndex
  • DiffDelta
  • DiffFile
  • FetchHead
  • GitTree
  • GitDiff
  • GitAnnotated
  • GitRebase
• Better documentation (#18810)
  • what the function/type does
  • arguments/output value
  • equivalent git CLI command where appropriate
• Better tests:
  • More coverage
  • Can test behaviour against command line Git
• Consistent handling of git_*_options structs
  • Correct handling of memory allocation of pointer fields
  • Callback interface
• Rename isdiff to a more descriptive name (https://github.com/JuliaLang/julia/pull/20155#discussion_r97181726)
• get rid of cat?
  • It's more or less a wrapper around content.
• document status/GitStatus return types and what results mean https://github.com/JuliaLang/julia/pull/20503#issuecomment-278394964

Some of these are breaking, so we may want to get the deprecations in place before 0.6.