Use proper constants for .gitignore and .git directory

Use proper constants for .gitignore and .git directory

We have a constant for .gitignore, so use it.  While we are in
the same method, correct the reference of ".git" to be the actual
GIT_DIR given.  This might not be within the work tree if the
GIT_DIR and GIT_WORK_TREE environment variables were used.

Change-Id: I38e1cec13405109b9c347858b38dd9fb2f1f2560
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
CC: Charley Wang <chwang@redhat.com>
CC: Chris Aniszczyk <caniszczyk@gmail.com>
CC: Stefan Lay <stefan.lay@sap.com>
CC: Matthias Sohn <matthias.sohn@sap.com>