### Please describe the problem.
git annex on windows does not seem to be able to get files from one (local) repository to another (also local) if the remote contains a drive letter (or generally a : in the path)

### What steps will reproduce the problem?
0. c:\> git init annex1
0. c:\> cd annex1
0. c:\annex1\> git annex init dir1
0. c:\annex1\> echo "This is a git annex repository" > README.txt
0. c:\annex1\> git annex add README.txt
0. c:\annex1\> git annex sync
0. c:\annex1\> cd \
1. c:\> git init annex2
1. c:\> cd annex2
1. c:\annex2\> git annex init dir2
1. c:\annex2\> git remote add dir1 c:\annex1
1. c:\annex2\> git annex sync dir1
2. c:\annex2\> git annex get README.txt

### What version of git-annex are you using? On what operating system?
C:\annex2>git version
git version 1.9.4.msysgit.0

C:\annex2>git annex version
git-annex version: 5.20150113-gcf247cf
build flags: Assistant Webapp Webapp-secure Pairing Testsuite S3 WebDAV DNS Feed
s Quvi TDFA CryptoHash TorrentParser
key/value backends: SHA256E SHA1E SHA512E SHA224E SHA384E SKEIN256E SKEIN512E SH
A256 SHA1 SHA512 SHA224 SHA384 SKEIN256 SKEIN512 WORM URL
remote types: git gcrypt S3 bup directory rsync web bittorrent webdav tahoe glac
ier ddar hook external
local repository version: 5
supported repository version: 5
upgrade supported from repository versions: 2 3 4


### Please provide any additional information below.

[[!format sh """
C:\>git init annex1
Initialized empty Git repository in C:/annex1/.git/

C:\>cd annex1

C:\annex1>git annex init dir1
init dir1
  Detected a filesystem without fifo support.

  Disabling ssh connection caching.

  Detected a crippled filesystem.

  Enabling direct mode.
ok
(Recording state in git...)

C:\annex1>echo "This is a git annex repository" > README.txt

C:\annex1>git annex add README.txt
add README.txt ok
(Recording state in git...)

C:\annex1>git annex sync
commit  ok

C:\annex1>cd \

C:\>git init annex2
Initialized empty Git repository in C:/annex2/.git/

C:\>cd annex2

C:\annex2>git annex init dir2
init dir2
  Detected a filesystem without fifo support.

  Disabling ssh connection caching.

  Detected a crippled filesystem.

  Enabling direct mode.
ok
(Recording state in git...)

C:\annex2>git remote add dir1 c:\annex1

C:\annex2>git annex sync dir1
commit  ok
pull dir1
warning: no common commits
remote: Counting objects: 13, done.
remote: Compressing objects: 100% (9/9), done.
remote: Total 13 (delta 1), reused 0 (delta 0)
Unpacking objects: 100% (13/13), done.
From c:\annex1
 * [new branch]      annex/direct/master -> dir1/annex/direct/master
 * [new branch]      git-annex  -> dir1/git-annex
 * [new branch]      master     -> dir1/master
 * [new branch]      synced/master -> dir1/synced/master

Merge made by the 'recursive' strategy.
 README.txt | 1 +
 1 file changed, 1 insertion(+)
 create mode 120000 README.txt

Already up-to-date.
ok
(merging dir1/git-annex into git-annex...)
(Recording state in git...)
push dir1
Counting objects: 15, done.
Delta compression using up to 4 threads.
Compressing objects: 100% (10/10), done.
Writing objects: 100% (12/12), 1.18 KiB | 0 bytes/s, done.
Total 12 (delta 4), reused 0 (delta 0)
To c:\annex1
   a7d2b83..0e86493  annex/direct/master -> synced/master
 * [new branch]      git-annex -> synced/git-annex
ok

C:\annex2>git annex get README.txt
get README.txt (not available)
  Try making some of these repositories available:
        f005c222-3e80-46a3-81a2-72c6cae18035 -- dir1              <<<---- WTF: It's c:\annex1
failed
git-annex: get: 1 failed

C:\annex2>git annex list
here
|dir1
||web
|||bittorrent
||||
____ README.txt                                                   <<<----- WTF2: Why doesn't annex2 know that annex1 has a copy?

C:\annex2>git annex whereis README.txt
whereis README.txt (1 copy)
        f005c222-3e80-46a3-81a2-72c6cae18035 -- dir1
ok

C:\annex2>cd \annex1

C:\annex1>git annex list
(merging synced/git-annex into git-annex...)
here
|web
||bittorrent
|||
X__ README.txt                                                    <<<--- But annex1 knows where it is.

C:\annex1>git annex sync
commit  ok
merge synced/master
Updating a7d2b83..0e86493
Fast-forward
error: duplicate parent 0e86493f9431d6df13ef49831e00b22be93e509c ignored              <<<---- Could this be the problem?
ok

C:\annex1>cd \annex2

"""]]

> [[fixed|done]]; a simple path calculation bug. --[[Joey]]
