bin/get-extra-pick-list: use git merge-base to get the branchpoint
bin/get-extra-pick-list: use git merge-base to get the branchpoint

Since mesa development history is linear and the only diversion is at
the branchpoint. Thus we can drop the ad-hoc parsing and use git
merge-base to retrieve it.

Cc: "13.0 17.0" <59f39c0db42d4479a46b02d4d2bc11120e37bb44@lists.freedesktop.org>
Signed-off-by: Emil Velikov <f20878473f414da183da19ee25c8722cc1d711e4@collabora.com>
Reviewed-by: Eric Engestrom <de0b63cf2dfb1fae8ed3c3b4244856a05677d4f0@imgtec.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler