Tabify indentations
index: Enhance documentation
index: Make _read() cope with index file creation
Merge pull request #1892 from libgit2/ntk/topic/index_read

Make git_index_read() cope with external additions and removals of the index file
Add test for multiple thread init/shutdown
Posix synchronized init, prototype win32 version
Use InterlockedCompareExchange for the lock
Don't use git_atomic as an integer