classes/buildhistory: do git garbage collection after committing

classes/buildhistory: do git garbage collection after committing

We don't normally perform any operations (such as "git pull") that
trigger "git gc --auto", thus garbage collection never happens which
means performance of accessing the repository degrades noticeably over
time. Add an explicit "git gc --auto" to clean things up when needed.

Thanks to Elijah Newren and Ross Burton for suggesting this.

(From OE-Core rev: 4a45a999e0ad2e99581428a5a6d34f483c00544f)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>