Revise parser to maintain its buffers between calls, so it should do fewer allocations.

git-svn-id: https://svn.r-project.org/R/trunk@63005 00db46b3-68df-0310-9c12-caf00c1e9a41
Missed check in r63005

git-svn-id: https://svn.r-project.org/R/trunk@63006 00db46b3-68df-0310-9c12-caf00c1e9a41
Back out r63005 and r63006

git-svn-id: https://svn.r-project.org/R/trunk@63007 00db46b3-68df-0310-9c12-caf00c1e9a41
add incoming check for BuildVignettes in FOSS packages

git-svn-id: https://svn.r-project.org/R/trunk@63008 00db46b3-68df-0310-9c12-caf00c1e9a41