fixture: git alternates

[darkowlzz]

This adds a new worktree with 2 repos, rep1 and rep2. rep2 is a shared
repo of rep1 and contains a relative alternates path to rep1. This would
be used to test git alternates.

I didn't find any instructions on adding new fixture and tried to figure out by myself.

• Created a git repo, rep1. Made an empty commit. Created a shared repo rep2 at the same directory level as rep1. By default, the .git/objects/info/alternates contains absolute path to the source repo's object directory. Changed that to a relative path ../../../rep1//.git/objects.
• Obtained a hash by doing:

$ find rep1/ rep2/ -type f | sha1sum
a6b6ff89c593f042347113203ead1c14ab5733ce  -

• Compressed the two repos with:

$ tar -zcvf worktree-a6b6ff89c593f042347113203ead1c14ab5733ce.tgz rep1 rep2

• Placed this compressed file in the data/ directory.
• Added a Fixture for the worktree in fixtures.go.

I made all these changes in my $GOPATH. Running the corresponding test for git alternates in go-git repo used the above created fixture and ran the test as expected. The test is at src-d/go-git#663 .

[mcuadros]

can you rebase? sorry