Fix download strategy for SVN HEAD
Fix download strategy for SVN HEAD

After converting to Pathname to create paths, using '+=' will result in
path concatenation by '/', which result in weird path like
'gcc--svn/-HEAD'. This patch should fix this.

Closes Homebrew/homebrew#19233.

Signed-off-by: Jack Nagel <43386ce32af96f5c56f2a88e458cb94cebee3751@gmail.com>

tonyg/homebrew,hanxue/linuxbrew,maxim-belkin/brew,reitermarkus/brew,nandub/brew,mistydemeo/homebrew,mistydemeo/homebrew,aw1621107/brew,sjackman/homebrew,ilovezfs/brew,claui/brew,vitorgalvao/brew,tonyg/homebrew,AnastasiaSulyagina/brew,MikeMcQuaid/brew,konqui/brew,reelsense/brew,gregory-nisbet/brew,AnastasiaSulyagina/brew,gordonmcshane/homebrew,mahori/brew,mahori/brew,Homebrew/brew,bfontaine/brew,muellermartin/dist,vitorgalvao/brew,hanxue/linuxbrew,maxim-belkin/brew,muellermartin/dist,toonetown/homebrew,gordonmcshane/homebrew,palxex/brew,mahori/brew,JCount/brew,sjackman/homebrew,zmwangx/brew,rwhogg/brew,claui/brew,Homebrew/brew,palxex/brew,amar-laksh/brew_sudo,gregory-nisbet/brew,JCount/brew,zmwangx/brew,EricFromCanada/brew,mgrimes/brew,konqui/brew,nandub/brew,AnastasiaSulyagina/brew,pseudocody/brew,maxim-belkin/brew,reitermarkus/brew,alyssais/brew,DomT4/brew,Linuxbrew/brew,bfontaine/brew,EricFromCanada/brew,zmwangx/brew,vitorgalvao/brew,konqui/brew,palxex/brew,jmsundar/brew,Linuxbrew/brew,JCount/brew,amar-laksh/brew_sudo,toonetown/homebrew,mistydemeo/homebrew,gordonmcshane/homebrew,MikeMcQuaid/brew,vitorgalvao/brew,amar-laksh/brew_sudo,jmsundar/brew,jmsundar/brew,DomT4/brew,EricFromCanada/brew,MikeMcQuaid/brew,JCount/brew,reitermarkus/brew,alyssais/brew,Linuxbrew/brew,pseudocody/brew,aw1621107/brew,konqui/brew,muellermartin/dist,reelsense/brew,bfontaine/brew,Homebrew/brew,Linuxbrew/brew,nandub/brew,toonetown/homebrew,rwhogg/brew,mahori/brew,staticfloat/brew,EricFromCanada/brew,staticfloat/brew,ilovezfs/brew,reitermarkus/brew,staticfloat/brew,mgrimes/brew,nandub/brew,MikeMcQuaid/brew,Homebrew/brew,gregory-nisbet/brew,tonyg/homebrew,pseudocody/brew,claui/brew,rwhogg/brew,DomT4/brew,ilovezfs/brew,hanxue/linuxbrew,sjackman/homebrew,DomT4/brew,aw1621107/brew,reelsense/brew,alyssais/brew,mgrimes/brew,claui/brew,sjackman/homebrew