Discovery path v2

Repository: Added the git_repository_discover function that finds by itself the git directory that manage a given directory path.

Repository: Splitted the repository destructor into a helper part (only free directories path) and the complete public destructor.

Repository: Added read_gitfile that allows you to read a .git file and extract the git directory path.

retrieve_device returns the file device for a given path (so that we can detect device change while walking through parent directories).
abspath returns a canonicalized path, symbolic link free.
retrieive_ceiling_directories_offset returns the biggest path offset that path match in the ceiling directory list (so that we can stop at ceiling directories).

Added gitfo_creat_locked and gitfo_creat_locked_force

As suggested by Romain-Geissler

Conflicts:

	src/fileops.c

Fixed a Windows TO-DO in the prettifying functions.

Fixed an issue with the `strtokz implementation and added support for
comments and relative paths in the alternates file.

Fileops:retrieve_path_root_offset is now named gitfo_retrieve_path_root_offset (like other public functions). Added platform specific directory separator definition.

Fix fake wstream write

Fixed some error messages related to searching objects from a short oid. Fixed forgot to check that prefix length is greater than minimum prefix length in read_unique_short_oid method from pack backend.

Deleted unused method git_cached_object_match, since we do not explore the cache when searching objects from a short oid.

Changed return value of git_oid_match to be consistent with the other compare methods (0 means oids match). Added method to compare prefixes of hex formatted oids.

Added a GIT_OID_MINPREFIXLEN constant to define the minimum length allowed for oid prefixes (set to 4, like in git). Consequently updated some object lookup methods and their documentation.

Implemented find_unique_short_oid for pack backend, based on git sha1 lookup method;
finding an object given its full oid is just a particular case of searching
the unique object matching an oid prefix (short oid).

Added git_odb_read_unique_short_oid, which iterates over all the backends to
find and read the unique object matching the given oid prefix.

Added a git_object_lookup_short_oid method to find the unique object in
the repository matching a given oid prefix : it generalizes git_object_lookup
which now does nothing but calls git_object_lookup_short_oid.

Added error for ambiguous oid prefixes. Added methods to compare the first nth hexadecimal characters (i.e. packets of 4 bits) of OIDs.

Added a read_unique_short_oid method to backends, to make it possible to find objects from sha1 prefixes in the future. Default implementations throw GIT_ENOTIMPLEMENTED for strict prefixes (i.e. length < GIT_OID_HEXSZ).

Fix tag and signature parsing

Fix typos

test_lib.c: flush stdout after every test-run

Plug a leak in the index unmerged entries vector

Config bugfix

Add git_repository_is_bare() accessor

A TREE extension with an entry count of -1 means that it was
invalidated and we should ignore it. Do so instead of returning an
error.

This fixes issue #202

Signed-off-by: Carlos Martín Nieto <cmn@elego.de>

There are two reasons why read_tree_internal might return a NULL
tree. The first one is a corrupt index, but the second one is an
invalidated TREE extension. Up to now, its only way to communicate
with its caller was through the return value being NULL or not.

Allow read_tree_internal to report its exit status independently from
the tree pointer.

Signed-off-by: Carlos Martín Nieto <cmn@elego.de>

The `git_repository_index` call now returns a brand new index that must
be manually free'd.

Before this commit, malformed tag and signature were considered as
valid by the parser. See the test t3800-mktag.sh of git to see example
of malformed tag and signature.

Signed-off-by: Carlos Martín Nieto <cmn@elego.de>