reference_rename: respect all references v2

Cleanup the thread-utils file. Do not define TLS if libgit2 is not
threadsafe.

[Discovery path] Fix and tests

Repository: A little fix in error code. GIT_ENOTFOUND is returned when a gitfile is malformed and GIT_ENOTAREPO when the pointed dir is not a repo.
Fixed tests so that it check the right error code.

The filebuf was not being properly written after a flush. This should
cut it now.

Fixes #228

the error returned by the subsystem; clarify error message.

Fix tiny typo.

Signed-off-by: schu <schu-github@schulog.org>

Add a new function reference_available() to check if a reference name
actually is free and can be used.

Signed-off-by: schu <schu-github@schulog.org>

Signed-off-by: schu <schu-github@schulog.org>

Unfortunately, the across_fs flag can't be tested automaticly, as we can't
create a temporary new filesystem.

It used to discard the last directory if the path didn't have a trailing slash.

Now the ceiling_dirs are compared with their symbolic free version (like base_path).
The ceiling dirs check is now performed after getting the parent directory.

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.