Don't save the commit buffer in git-describe.

The commit buffer (message of the commit) is not actually
used by the git-describe process.  We can save some memory
by not keeping it around.
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

describe.c

@@ -162,6 +162,7 @@ int main(int argc, char **argv)
 	}
 
 	setup_git_directory();
+	save_commit_buffer = 0;
 
 	if (argc <= i)
 		describe("HEAD", 1);