tools/lib/git/git.go:  Change git.BranchesDiffer implementation so it only looks at the names of changed files.
tools/lib/git/git.go:  Change git.BranchesDiffer implementation so it
only looks at the names of changed files.

git veyron review was crashing on files with really (really really) long
lines because it calls "git diff" to see what the changes are, and a
buffer overflow was happening inside cmd.Run.  Now we only look at the
names of changed files.

Change-Id: Id6b85398e2f7f04df4270db10f948b11716bfc11

vanadium/go.devtools,vanadium/go.devtools,vanadium/go.devtools