Search (git grep) does not handle long lines

[beyang]

Because go-vcs uses bufio.Scanner to read lines from git output (see the Search method in vcs/gitcmd/repo.go), it fails on super long lines.

It should more gracefully handle this error or use bufio.Reader.ReadLine.

Repro: try searching for "foo" in the repository github.com/treeio/treeio (this returns some long lines of minified JSON).