BlameGenerator: Use cache only for candidates modifying the path

BlameGenerator is querying the cache on each candidate from the
queue. i.e. on every commit in the history that contains the path. The
cache call is expensive and, in long histories with poor cache
coverage, the blame becomes slower than without cache.

Query the cache afer we know the candidate modified the file but
before doing the actual blame. In other words, ignore the cache if the
commit doesn't modify the file.

Compared with previous code, this skips the cache for the commit that
creates the file and for the root commit (blame is that commit, so no
performance loss).

Change-Id: I0fd7279026a30505742527e84f13680b843ad4a3

0 files changed, 0 insertions, 0 deletions