keg: don't delete main opt links of other formulae.
keg: don't delete main opt links of other formulae.

If an old alias, according to the tab, of a formula being upgraded (e.g.
python) has the same name as the main opt link of some other formula
(e.g. python@2), don't remove the main opt link of that other formula.

Specifically,

  brew install python@2
  brew upgrade python

should not accidentally delete /usr/local/opt/python@2 even though
python@2 used to be an alias of python.

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