Fixed an error where "nonexistant^N" or similar
would fall into an assert. This now properly returns
an error. 

Ported date.c (for approxidate_careful) from git.git
revision aa39b85. Trimmed out the parts we're not
using.

Added tracking configuration to the test repo's
config to support unit tests.

Removed repeated strlen's, and unnecessary loop-termination variable.

in the test repo.

Added some reflags to the test repo to support
unit tests.

Also extended the test suite to include chaining
operators, e.g. "master^2~3^4".

Supported forms:
- "^n"
- "^0"
- "^"

Still missing: all of the "^{…}" variants.

New tests for "foo^2" syntax, but they don't pass
yet. Support for chaining these, i.e.
"foo^2~3^{u}~1' is starting to shape up.

This version supports refspecs of these kinds:
- Full & partial SHAs
- Output from "git describe"
- "/refs/heads/master" (full ref names)
- "master" (partial ref names)
- "FETCH_HEAD" (named heads)

Use giterr_last() and make sure it's not NULL.

Fix MSVC compilation issue

    exp() is already defined in math.h. This leads to LMSVC complaining

    ..\..\libgit2\tests-clar\diff\blob.c(5): error C2365: 'exp' : redefinition; previous definition was 'function'

    Renaming the variable fixes this issue.

util: Fix git__isspace() implementation

These objects aren't considered as being advertised, so asking for
them will cause the remote end to close the connection. This makes the
checking in update_tips() unnecessary, because they don't get inserted
in the list.

Skip them for now. Eventually we might want to filter these out
earler.

The characters <space>, <form-feed>, <newline>, <carriage-return>, <tab>, and <vertical-tab> are part of the "space" definition.

cf. http://www.kernel.org/doc/man-pages/online/pages/man5/locale.5.html

Windows gets upset if we rename a file with an open descriptor.