[git] Be more specific when looking for llvm-svn
A commit may, for some reason, have llvm-svn: in it multiple times. It may even take up the whole line and look identical to what gets added automatically when svn commits land in github.
To workaround this, make changes to both lookups:
- When doing the git -> svn lookup, make sure to go through the whole message, and: a) Only look for llvm-svn starting at the beginning of the line (excluding the whitespace that git log adds). b) Take the last one (at the end of the commit message), if there are multiple matches.
- When doing the svn -> git lookup, look through a sizeable but still reasonably small number of git commits (10k, about 4-5 months right now), and: a) Only consider commits with the '^llvm-svn: NNNNNN' we expect, and b) Only consider those that also follow the same git -> svn matching above. (Error if it's not exactly one commit).
Reviewed By: jyknight
Differential Revision: https://reviews.llvm.org/D60017