Add contrib/rerere-train script
Add contrib/rerere-train script

This script takes a range of commits (e.g. maint..next) as its arguments,
recreates merge commits in the range to prime rr-cache database.

Signed-off-by: Nanako Shiraishi <7d97fbf70141dda63063292abadb698cc511a125@lavabit.com>
Signed-off-by: Shawn O. Pearce <f01032651c12a22b2103346ba96b3b6b231f9d3c@spearce.org>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git