Change Memo-backing map to a Guava LoadingCache

MapMaker().makeComputingMap() is deprecated, due for removal in Feb
2013! So even though our use here is more like a map, and doesn't ever
want eviction, best to switch, and use the default config of no
automatic-eviction.

Perf seems identical cleaning against the R2 codebase - 52s for both.


$ java -jar bfg-0.6.0.jar -B 10 r2.git
CMDConfig(Some(10),None,Set(HEAD),*,List(),List(),Some(/home/roberto/development/blob-fixing-git-cleaner-debugging/r2.git))
Using repo : /home/roberto/development/blob-fixing-git-cleaner-debugging/r2.git
Found 10408 objects to protect
Found 10 blob ids to remove biggest=68299693 smallest=10068435
Total size (unpacked)=240017842
Finding target blobs completed in 6,379 ms.
Found 260 commit-pointing refs
Getting full commit list:
Found 48196 commits
Cleaning commits:       100% (48196/48196)
Cleaning commits completed in 51,991 ms.


$ java -jar /home/roberto/development/bfg-repo-cleaner/target/bfg-repo-cleaner-assembly-0.7.0-SNAPSHOT.jar -B 10 r2.git
CMDConfig(Some(10),None,Set(HEAD),*,List(),List(),Some(/home/roberto/development/blob-fixing-git-cleaner-debugging/r2.git))
Using repo : /home/roberto/development/blob-fixing-git-cleaner-debugging/r2.git
Found 10408 objects to protect
Found 10 blob ids to remove biggest=68299693 smallest=10068435
Total size (unpacked)=240017842
Finding target blobs completed in 7,129 ms.
Found 260 commit-pointing refs
Getting full commit list:
Found 48196 commits
Cleaning commits:       100% (48196/48196)
Cleaning commits completed in 52,012 ms.