Benchmark_Driver: Support custom baseline branches
Benchmark_Driver: Support custom baseline branches

Support specifying a baseline branch to compare the current results
against. Previously, the master branch was hardcoded.

Fixes: rdar://problem/32751587

tjw/swift,tkremenek/swift,parkera/swift,return/swift,allevato/swift,austinzheng/swift,swiftix/swift,felix91gr/swift,natecook1000/swift,amraboelela/swift,zisko/swift,tkremenek/swift,apple/swift,djwbrown/swift,uasys/swift,return/swift,shajrawi/swift,apple/swift,djwbrown/swift,shahmishal/swift,nathawes/swift,return/swift,lorentey/swift,aschwaighofer/swift,jopamer/swift,parkera/swift,austinzheng/swift,nathawes/swift,practicalswift/swift,harlanhaskins/swift,frootloops/swift,frootloops/swift,frootloops/swift,sschiau/swift,jckarter/swift,benlangmuir/swift,lorentey/swift,stephentyrone/swift,gribozavr/swift,danielmartin/swift,jopamer/swift,gribozavr/swift,hughbe/swift,allevato/swift,karwa/swift,sschiau/swift,nathawes/swift,CodaFi/swift,tkremenek/swift,JGiola/swift,airspeedswift/swift,allevato/swift,jckarter/swift,austinzheng/swift,danielmartin/swift,parkera/swift,OscarSwanros/swift,shahmishal/swift,stephentyrone/swift,JGiola/swift,atrick/swift,jopamer/swift,apple/swift,CodaFi/swift,practicalswift/swift,jopamer/swift,karwa/swift,ahoppen/swift,tjw/swift,sschiau/swift,aschwaighofer/swift,danielmartin/swift,devincoughlin/swift,aschwaighofer/swift,JGiola/swift,parkera/swift,ahoppen/swift,lorentey/swift,shajrawi/swift,benlangmuir/swift,felix91gr/swift,djwbrown/swift,alblue/swift,roambotics/swift,amraboelela/swift,milseman/swift,felix91gr/swift,sschiau/swift,xedin/swift,xwu/swift,atrick/swift,practicalswift/swift,airspeedswift/swift,aschwaighofer/swift,swiftix/swift,jmgc/swift,devincoughlin/swift,aschwaighofer/swift,karwa/swift,OscarSwanros/swift,gribozavr/swift,CodaFi/swift,lorentey/swift,frootloops/swift,uasys/swift,xwu/swift,austinzheng/swift,xwu/swift,allevato/swift,CodaFi/swift,hooman/swift,rudkx/swift,zisko/swift,swiftix/swift,ahoppen/swift,shahmishal/swift,tkremenek/swift,amraboelela/swift,sschiau/swift,CodaFi/swift,deyton/swift,alblue/swift,glessard/swift,OscarSwanros/swift,djwbrown/swift,alblue/swift,hooman/swift,OscarSwanros/swift,austinzheng/swift,gregomni/swift,felix91gr/swift,felix91gr/swift,devincoughlin/swift,frootloops/swift,JGiola/swift,ahoppen/swift,hooman/swift,hughbe/swift,hooman/swift,tkremenek/swift,hooman/swift,xedin/swift,devincoughlin/swift,nathawes/swift,xedin/swift,swiftix/swift,tjw/swift,parkera/swift,harlanhaskins/swift,lorentey/swift,return/swift,stephentyrone/swift,tjw/swift,nathawes/swift,gribozavr/swift,gregomni/swift,apple/swift,karwa/swift,swiftix/swift,shajrawi/swift,gregomni/swift,austinzheng/swift,return/swift,amraboelela/swift,shajrawi/swift,CodaFi/swift,rudkx/swift,alblue/swift,hughbe/swift,gregomni/swift,jmgc/swift,OscarSwanros/swift,deyton/swift,felix91gr/swift,jmgc/swift,benlangmuir/swift,gregomni/swift,rudkx/swift,huonw/swift,jmgc/swift,practicalswift/swift,glessard/swift,shajrawi/swift,jmgc/swift,xedin/swift,lorentey/swift,amraboelela/swift,deyton/swift,aschwaighofer/swift,xwu/swift,lorentey/swift,apple/swift,jmgc/swift,milseman/swift,huonw/swift,airspeedswift/swift,jmgc/swift,stephentyrone/swift,tjw/swift,natecook1000/swift,hughbe/swift,deyton/swift,hughbe/swift,roambotics/swift,allevato/swift,allevato/swift,hooman/swift,atrick/swift,practicalswift/swift,hughbe/swift,practicalswift/swift,practicalswift/swift,zisko/swift,OscarSwanros/swift,apple/swift,milseman/swift,parkera/swift,natecook1000/swift,deyton/swift,return/swift,devincoughlin/swift,alblue/swift,devincoughlin/swift,brentdax/swift,danielmartin/swift,zisko/swift,amraboelela/swift,huonw/swift,natecook1000/swift,harlanhaskins/swift,stephentyrone/swift,practicalswift/swift,harlanhaskins/swift,swiftix/swift,xedin/swift,glessard/swift,swiftix/swift,gribozavr/swift,glessard/swift,shahmishal/swift,huonw/swift,atrick/swift,atrick/swift,shahmishal/swift,xedin/swift,rudkx/swift,devincoughlin/swift,hughbe/swift,huonw/swift,zisko/swift,harlanhaskins/swift,brentdax/swift,natecook1000/swift,rudkx/swift,glessard/swift,frootloops/swift,stephentyrone/swift,uasys/swift,tjw/swift,hooman/swift,jckarter/swift,shahmishal/swift,benlangmuir/swift,sschiau/swift,jopamer/swift,ahoppen/swift,brentdax/swift,allevato/swift,rudkx/swift,atrick/swift,roambotics/swift,xedin/swift,OscarSwanros/swift,danielmartin/swift,airspeedswift/swift,benlangmuir/swift,harlanhaskins/swift,amraboelela/swift,tkremenek/swift,alblue/swift,jckarter/swift,djwbrown/swift,parkera/swift,milseman/swift,djwbrown/swift,shajrawi/swift,roambotics/swift,benlangmuir/swift,jopamer/swift,ahoppen/swift,devincoughlin/swift,airspeedswift/swift,alblue/swift,shahmishal/swift,JGiola/swift,jopamer/swift,tjw/swift,xedin/swift,uasys/swift,roambotics/swift,xwu/swift,felix91gr/swift,milseman/swift,nathawes/swift,austinzheng/swift,CodaFi/swift,lorentey/swift,karwa/swift,deyton/swift,parkera/swift,danielmartin/swift,karwa/swift,roambotics/swift,return/swift,deyton/swift,nathawes/swift,jckarter/swift,jckarter/swift,gregomni/swift,airspeedswift/swift,brentdax/swift,brentdax/swift,uasys/swift,jckarter/swift,uasys/swift,shahmishal/swift,tkremenek/swift,karwa/swift,aschwaighofer/swift,sschiau/swift,djwbrown/swift,glessard/swift,frootloops/swift,shajrawi/swift,karwa/swift,brentdax/swift,xwu/swift,JGiola/swift,milseman/swift,shajrawi/swift,uasys/swift,xwu/swift,gribozavr/swift,gribozavr/swift,milseman/swift,huonw/swift,natecook1000/swift,harlanhaskins/swift,sschiau/swift,huonw/swift,zisko/swift,natecook1000/swift,airspeedswift/swift,gribozavr/swift,brentdax/swift,zisko/swift,danielmartin/swift,stephentyrone/swift