utils/git: use exact format for last_revision_*
utils/git: use exact format for last_revision_*

Format for oneline can be overridden in user's gitconfig. If that's the
case, last_revision_commit_of_file won't work properly, and because of
that last_revision_of_file won't work at all.

Here's what I was getting when running brew tests:

       expected: "6bec2de"
            got: "\e[33m6bec2dee633f\e[m"

I have abbrev length set to 12, and oneline formatting is more colorful.

So, instead of relying on overrideable format, it should rather specify
"--format=%h" to get only hash, and speciy --abbrev=7 to force abbrev
length to be 7

After this commit tests are passing for me.

reitermarkus/brew,mahori/brew,Linuxbrew/brew,MikeMcQuaid/brew,DomT4/brew,sjackman/homebrew,vitorgalvao/brew,reitermarkus/brew,konqui/brew,konqui/brew,vitorgalvao/brew,mgrimes/brew,DomT4/brew,MikeMcQuaid/brew,JCount/brew,DomT4/brew,maxim-belkin/brew,MikeMcQuaid/brew,maxim-belkin/brew,ilovezfs/brew,EricFromCanada/brew,EricFromCanada/brew,ilovezfs/brew,reitermarkus/brew,mahori/brew,mahori/brew,reitermarkus/brew,Homebrew/brew,claui/brew,konqui/brew,mahori/brew,claui/brew,DomT4/brew,claui/brew,nandub/brew,JCount/brew,konqui/brew,hanxue/linuxbrew,Homebrew/brew,MikeMcQuaid/brew,hanxue/linuxbrew,hanxue/linuxbrew,sjackman/homebrew,mgrimes/brew,Homebrew/brew,JCount/brew,Homebrew/brew,sjackman/homebrew,claui/brew,sjackman/homebrew,maxim-belkin/brew,Linuxbrew/brew,EricFromCanada/brew,mgrimes/brew,vitorgalvao/brew,nandub/brew,Linuxbrew/brew,EricFromCanada/brew,JCount/brew,ilovezfs/brew,vitorgalvao/brew,nandub/brew,Linuxbrew/brew,nandub/brew