Don't run "git fetch" multiple times for the same repository.

Don't run "git fetch" multiple times for the same repository.

When looking into issue 17854, I noticed that when installing without a locked
git revision, "pub get" was running "git fetch" over and over again on the same
repository. This avoids doing so and increases the speed of "pub get" with git
repositories considerably.

R=rnystrom@google.com

Committed: https://code.google.com/p/dart/source/detail?r=34671

[Bob Nystrom, 2014-04-02 21:03:24]

LGTM. Very nice.

[nweiz, 2014-04-02 21:05:58]

Committed patchset #1 manually as r34671 (presubmit successful).