Merge pull request #326 from libgit2/null-index-entries

Index entries could be NULL.
Fix `userSignatureForNow` sometimes returning nil.

On the iOS simulator, `NSFullUserName()` (and presumably
`NSUserName()`) return an empty string; currently we check for nil.
This leads to `userSignatureForNow` returning nil and, eventually, a
crash. Instead make sure that `NSFullUserName()` and `NSUserName()`
don’t return an empty string or nil.
Use a simpler check.
It's == not !=.
Repeated calls are wasteful.
Merge pull request #328 from PiersonBro/fix-userSignatureForNow

Fix `userSignatureForNow` sometimes returning nil.
Merge pull request #327 from libgit2/filters

Filter support
Merge pull request #330 from libgit2/flush-attrs-cache

Added -flushAttributesCache.
Stub APIs for GTFilterList
Import GTFilterList.h and fix its visibility
Fix forward declarations and imports
Implement -filterListWithPath:…
Implement filter list application
Test filter list loading
Test + fix applying a single filter
Test applying multiple filters to data
Don't require equivalent repositories.

Repository equivalence is Tricky® and if we can’t diff, we’ll find out
from libgit2.
Merge pull request #332 from libgit2/dont-require-same-repository

Don't require equivalent repositories.
Test applying a filter list to a file and a blob
Ensure both filters are applied, test combined output