Extract the keys for the (Git)ModifiedFilesCaches to separate classes

Change-Id: If8ea827f82ae369eb744fe8eb60e88f528762179
diff --git a/proto/cache.proto b/proto/cache.proto
index e7fbe6d..aa71b87 100644
--- a/proto/cache.proto
+++ b/proto/cache.proto
@@ -528,7 +528,7 @@
 }
 
 // Serialized key for
-// com.google.gerrit.server.patch.gitdiff.GitModifiedCacheImpl.Key
+// com.google.gerrit.server.patch.gitdiff.GitModifiedFilesCacheKey
 // Next ID: 5
 message GitModifiedFilesKeyProto {
   string project = 1;
@@ -537,6 +537,16 @@
   int32 rename_score = 4;
 }
 
+// Serialized key for
+// com.google.gerrit.server.patch.diff.ModifiedFilesCacheKey
+// Next ID: 5
+message ModifiedFilesKeyProto {
+  string project = 1;
+  bytes a_commit = 2; // SHA-1 hash of the left commit ID in the diff
+  bytes b_commit = 3; // SHA-1 hash of the right commit ID in the diff
+  int32 rename_score = 4;
+}
+
 // Serialized form of com.google.gerrit.server.patch.gitdiff.ModifiedFile
 // Next ID: 4
 message ModifiedFileProto {