New external functions:
	- git_index_unmerged_entrycount: Counts the unmerged entries in
	  the index
	- git_index_get_unmerged: Gets an unmerged entry from the index
	  by name

New internal functions:
	- read_unmerged: Wrapper for read_unmerged_internal
	- read_unmerged_internal: Reads unmerged entries from the index
	  if the index has the INDEX_EXT_UNMERGED_SIG set
	- unmerged_srch: Search function for unmerged vector
	- unmerged_cmp: Compare function for unmerged vector

New data structures:
	- git_index now contains a git_vector unmerged that stores
	  unmerged entries
	- git_index_entry_unmerged: Representation of an unmerged file
	  entry. It represents all three versions of the file at the
	  same time, with one name, three modes and three OIDs