Merge pull request #1810 from nvloff/reference_is_tag

refs: add git_reference_is_tag
Improve win32 version check, no ipv6 tests on XP
Merge pull request #1812 from ethomson/version_check

Windows XP updates
submoule: extract submodule_add tests
submodule: fix add_setup segfault for bare repos

Calling git_submodule_add_setup on a bare repo results in the following
segfault:

OS Version:      Mac OS X 10.8.4 (12E55)
Report Version:  10

Crashed Thread:  0  Dispatch queue: com.apple.main-thread

Exception Type:  EXC_BAD_ACCESS (SIGSEGV)
Exception Codes: KERN_INVALID_ADDRESS at 0x0000000000000000

VM Regions Near 0:
-->
    __TEXT                 000000010cdc2000-000000010d03f000 [ 2548K] r-x/rwx SM=COW  /Users/USER/*

Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
0   libgit2_clar                  	0x000000010ce6319e git__prefixcmp + 30 (util.c:346)
1   libgit2_clar                  	0x000000010ce55e2e git_submodule_add_setup + 638 (submodule.c:242)
2   libgit2_clar                  	0x000000010cf42f71 test_submodule_add__bare + 65 (add.c:34)
3   libgit2_clar                  	0x000000010ce7d637 clar_run_test + 103 (clar.c:161)
4   libgit2_clar                  	0x000000010ce7c61b clar_run_suite + 459 (clar.c:223)
5   libgit2_clar                  	0x000000010ce7c2e6 clar_parse_args + 710 (clar.c:280)
6   libgit2_clar                  	0x000000010ce7bd1a clar_test + 106 (clar.c:337)
7   libgit2_clar                  	0x000000010ce7ad7a main + 42 (main.c:14)
8   libdyld.dylib                 	0x00007fff94b7f7e1 start + 1

Thread 0 crashed with X86 Thread State (64-bit):
  rax: 0x0000000000000000  rbx: 0x0000000000000000  rcx: 0x0000000000000001  rdx: 0x0000000000000000
  rdi: 0x000000010cf64a1a  rsi: 0x0000000000000000  rbp: 0x00007fff52e3d750  rsp: 0x00007fff52e3d750
   r8: 0x0000000000000006   r9: 0x000000000fbe5c35  r10: 0x0000000000000000  r11: 0x00007f8a6a408820
  r12: 0x0000000000000000  r13: 0x0000000000000000  r14: 0x0000000000000000  r15: 0x0000000000000000
  rip: 0x000000010ce6319e  rfl: 0x0000000000010202  cr2: 0x0000000000000000
Logical CPU: 0
branch: add git_branch__current_remote_name

Gets the remote of the branch, that HEAD points to.
submodule: handle relative urls in submodule_add