[logs] Add new log messages

[mafesan]

After a first revision of the Git backend module, a few new log messages has been added:

• It may be useful to distinguish between parsing a given git log file or it has to obtain that log file from a repository.

logger.info("Fetching commits from the given log file %s", self.gitpath)
logger.info("Log file not specified, retrieving from repository.")

• As proposed, now a message informs if the repository is found locally or it has to be cloned first.

logger.info("Git repository not found in %s", self.gitpath)
logger.info("Found Git repository in %s", self.gitpath)

• Message logger.debug("Git %s repository cloned into %s",... has been changed into info type, as this information may help to discriminate if the program has stuck in the cloning phase.

[coveralls]

Coverage increased (+0.005%) to 95.736% when pulling b6b6938 on mghfdez:logs into ccc317c on grimoirelab:master.

[jgbarah]

I'm sorry this patch is now conflicting. I'm sorry this was sitting here for so long, but if you still think it is relevant, would you mind updtaing it to the latest version in the master branch?