init: plug tiny one-time memory leak
The buffer used to construct paths like ".git/objects/info" and
".git/objects/pack" is allocated on the heap and never freed.
So free it. While at it, factor out the relevant code into its own
function and rename the sha1_dir variable to object_directory (to
match the change in everyday usage after the renaming of
SHA1_FILE_DIRECTORY in v0.99~603^2~7, 2005).
Noticed by valgrind while setting up tests (in test-lib).
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>