*: skip time consuming tests

[mcuadros]

FIxes #780

I made a small review of the most time-consuming tests, and I skipped for -short executions. Now the root tests take around 5secs, and the whole project 42secs.

> env GOCACHE=off go test -short
ok  	gopkg.in/src-d/go-git.v4	9.591s
?   	gopkg.in/src-d/go-git.v4/cli/go-git	[no test files]
ok  	gopkg.in/src-d/go-git.v4/config	0.063s
ok  	gopkg.in/src-d/go-git.v4/internal/revision	0.041s
ok  	gopkg.in/src-d/go-git.v4/plumbing	0.016s
ok  	gopkg.in/src-d/go-git.v4/plumbing/cache	0.066s
ok  	gopkg.in/src-d/go-git.v4/plumbing/filemode	0.027s
ok  	gopkg.in/src-d/go-git.v4/plumbing/format/config	0.052s
ok  	gopkg.in/src-d/go-git.v4/plumbing/format/diff	0.045s
ok  	gopkg.in/src-d/go-git.v4/plumbing/format/gitignore	0.041s
ok  	gopkg.in/src-d/go-git.v4/plumbing/format/idxfile	0.290s
ok  	gopkg.in/src-d/go-git.v4/plumbing/format/index	0.090s
ok  	gopkg.in/src-d/go-git.v4/plumbing/format/objfile	0.059s
ok  	gopkg.in/src-d/go-git.v4/plumbing/format/packfile	0.725s
ok  	gopkg.in/src-d/go-git.v4/plumbing/format/pktline	0.123s
ok  	gopkg.in/src-d/go-git.v4/plumbing/object	1.015s
ok  	gopkg.in/src-d/go-git.v4/plumbing/protocol/packp	0.060s
ok  	gopkg.in/src-d/go-git.v4/plumbing/protocol/packp/capability	0.071s
ok  	gopkg.in/src-d/go-git.v4/plumbing/protocol/packp/sideband	0.035s
ok  	gopkg.in/src-d/go-git.v4/plumbing/revlist	0.104s
ok  	gopkg.in/src-d/go-git.v4/plumbing/storer	0.034s
ok  	gopkg.in/src-d/go-git.v4/plumbing/transport	0.016s
ok  	gopkg.in/src-d/go-git.v4/plumbing/transport/client	0.030s
ok  	gopkg.in/src-d/go-git.v4/plumbing/transport/file	4.389s
ok  	gopkg.in/src-d/go-git.v4/plumbing/transport/git	8.065s
ok  	gopkg.in/src-d/go-git.v4/plumbing/transport/http	4.174s
?   	gopkg.in/src-d/go-git.v4/plumbing/transport/internal/common	[no test files]
ok  	gopkg.in/src-d/go-git.v4/plumbing/transport/server	0.180s
ok  	gopkg.in/src-d/go-git.v4/plumbing/transport/ssh	0.556s
?   	gopkg.in/src-d/go-git.v4/plumbing/transport/test	[no test files]
?   	gopkg.in/src-d/go-git.v4/storage	[no test files]
ok  	gopkg.in/src-d/go-git.v4/storage/filesystem	4.558s
ok  	gopkg.in/src-d/go-git.v4/storage/filesystem/internal/dotgit	0.524s
ok  	gopkg.in/src-d/go-git.v4/storage/memory	0.014s
?   	gopkg.in/src-d/go-git.v4/storage/test	[no test files]
ok  	gopkg.in/src-d/go-git.v4/utils/binary	0.011s
ok  	gopkg.in/src-d/go-git.v4/utils/diff	0.023s
ok  	gopkg.in/src-d/go-git.v4/utils/ioutil	0.013s
ok  	gopkg.in/src-d/go-git.v4/utils/merkletrie	0.026s
ok  	gopkg.in/src-d/go-git.v4/utils/merkletrie/filesystem	0.014s
ok  	gopkg.in/src-d/go-git.v4/utils/merkletrie/index	0.007s
ok  	gopkg.in/src-d/go-git.v4/utils/merkletrie/internal/frame	0.018s
ok  	gopkg.in/src-d/go-git.v4/utils/merkletrie/internal/fsnoder	0.056s
ok  	gopkg.in/src-d/go-git.v4/utils/merkletrie/noder	0.059s

[mvdan]

🎉