odb: add git_odb_foreach()

Go through each backend and list every objects that exists in
them. This allows fsck-like uses.
Add a couple of useful git_buf utilities

* `git_buf_rfind` (with tests and tests for `git_buf_rfind_next`)
* `git_buf_puts_escaped` and `git_buf_puts_escaped_regex` (with tests)
  to copy strings into a buffer while injecting an escape sequence
  (e.g. '\') in front of particular characters.
Add path utilities to resolve relative paths

This makes it easy to take a buffer containing a path with relative
references (i.e. .. or . path segments) and resolve all of those
into a clean path.  This can be applied to URLs as well as file
paths which can be useful.

As part of this, I made the drive-letter detection apply on all
platforms, not just windows.  If you give a path that looks like
"c:/..." on any platform, it seems like we might as well detect
that as a rooted path.  I suppose if you create a directory named
"x:" on another platform and want to use that as the beginning
of a relative path under the root directory of your repo, this
could cause a problem, but then it seems like you're asking for
trouble.
Adding unicode space to match crlf patterns

Adding 0x85 to `git__isspace` since we also look for that in filter.c
as a whitespace character.
Adding git_config_foreach_match() iteration fn

Adding a new config iteration function that let's you iterate
over just the config entries that match a particular regular
expression.  The old foreach becomes a simple use of this with
an empty pattern.

This also fixes an apparent bug in the existing `git_config_foreach`
where returning a non-zero value from the iteration callback was
not correctly aborting the iteration and the returned value was
not being propogated back to the caller of foreach.

Added to tests to cover all these changes.
Add flag to write gitlink on setting repo workdir

This added a flag to the `git_repository_set_workdir()` function
that enables generation of a `.git` gitlink file that links the
new workdir to the parent repository.  Essentially, the flag tells
the function to write out the changes to disk to permanently set
the workdir of the repository to the new path.

If you pass this flag as true, then setting the workdir to something
other than the default workdir (i.e. the parent of the .git repo
directory), will create a plain file named ".git" with the standard
gitlink contents "gitdir: <repo-path>", and also update the
"core.worktree" and "core.bare" config values.

Setting the workdir to the default repo workdir will clear the
core.worktree flag (but still permanently set core.bare to false).

BTW, the libgit2 API does not currently provide a function for
clearing the workdir and converting a non-bare repo into a bare one.
Fix missing NUL termination of buffer
tests: add test commit with angle brackets in the author name
revwalk: relax the parsing of the commit time
signature: prevent angle bracket usage in identity
revparse: simplify the parsing of described object
commit: reduce code duplication
commit: introduce git_commit_nth_gen_ancestor()
revparse: deploy git_commit_nth_gen_ancestor()
refs: readonly tests don't need a sandboxed repo
refs: introduce git_reference_remote_tracking_from_branch()
revparse: deploy git_reference_remote_tracking_from_branch()
Merge pull request #812 from arrbee/assorted-tweaks

Assorted goodies
Merge pull request #814 from nulltoken/topic/revparse-refac

Revparse refactoring: a start
Merge pull request #805 from nulltoken/fix/revwalk-email-parsing

Fix revwalk email parsing