Add 'git cache fetch' subcommand.
Add 'git cache fetch' subcommand.

If you're in a git checkout cloned from the git cache, this will:
  - Update the cache with the latest upstream commits.
  - Update the cwd with the latest commits from the cache.

For example:

> cd $HOME/workspace/chromium/src
> git cache fetch

R=agable@chromium.org,iannucci@chromium.org
BUG=

Review URL: https://codereview.chromium.org/440213005

git-svn-id: fd409f4bdeea2bb50a5d34bb4d4bfc2046a5a3dd@288140 0039d316-1c4b-4281-b951-d872f2087c98

Midrya/chromium,fanjunwei/depot_tools,Phonebooth/depot_tools,CoherentLabs/depot_tools,duanwujie/depot_tools,gcodetogit/depot_tools,airtimemedia/depot_tools,sarvex/depot-tools,SuYiling/chrome_depot_tools,michalliu/chromium-depot_tools,ajohnson23/depot_tools,hsharsha/depot_tools,azunite/chrome_build,Midrya/chromium,fanjunwei/depot_tools,liaorubei/depot_tools,hsharsha/depot_tools,Neozaru/depot_tools,gcodetogit/depot_tools,primiano/depot_tools,smikes/depot_tools,primiano/depot_tools,liaorubei/depot_tools,duanwujie/depot_tools,HackFisher/depot_tools,mlufei/depot_tools,disigma/depot_tools,azureplus/chromium_depot_tools,npe9/depot_tools,chinmaygarde/depot_tools,Neozaru/depot_tools,ajohnson23/depot_tools,npe9/depot_tools,HackFisher/depot_tools,azunite/chrome_build,duanwujie/depot_tools,Phonebooth/depot_tools,G-P-S/depot_tools,xuyuhan/depot_tools,HackFisher/depot_tools,liaorubei/depot_tools,SuYiling/chrome_depot_tools,withtone/depot_tools,G-P-S/depot_tools,G-P-S/depot_tools,fracting/depot_tools,cpanelli/-git-clone-https-chromium.googlesource.com-chromium-tools-depot_tools,SuYiling/chrome_depot_tools,duongbaoduy/gtools,michalliu/chromium-depot_tools,fracting/depot_tools,mlufei/depot_tools,michalliu/chromium-depot_tools,azunite/chrome_build,cpanelli/-git-clone-https-chromium.googlesource.com-chromium-tools-depot_tools,fanjunwei/depot_tools,withtone/depot_tools,primiano/depot_tools,eatbyte/depot_tools,azureplus/chromium_depot_tools,disigma/depot_tools,fracting/depot_tools,CoherentLabs/depot_tools,cpanelli/-git-clone-https-chromium.googlesource.com-chromium-tools-depot_tools,kaiix/depot_tools,hsharsha/depot_tools,aleonliao/depot_tools,npe9/depot_tools,aleonliao/depot_tools,smikes/depot_tools,sarvex/depot-tools,airtimemedia/depot_tools,Neozaru/depot_tools,fanjunwei/depot_tools,sarvex/depot-tools,smikes/depot_tools,sarvex/depot-tools,Neozaru/depot_tools,aleonliao/depot_tools,chinmaygarde/depot_tools,ajohnson23/depot_tools,smikes/depot_tools,kaiix/depot_tools,airtimemedia/depot_tools,airtimemedia/depot_tools,Midrya/chromium,chinmaygarde/depot_tools,npe9/depot_tools,michalliu/chromium-depot_tools,gcodetogit/depot_tools,withtone/depot_tools,disigma/depot_tools,G-P-S/depot_tools,HackFisher/depot_tools,mlufei/depot_tools,eatbyte/depot_tools,smikes/depot_tools,xuyuhan/depot_tools,liaorubei/depot_tools,Neozaru/depot_tools,Phonebooth/depot_tools,xuyuhan/depot_tools,duongbaoduy/gtools,duongbaoduy/gtools,Phonebooth/depot_tools,eatbyte/depot_tools,azureplus/chromium_depot_tools,kaiix/depot_tools,xuyuhan/depot_tools,eatbyte/depot_tools