gitignore: use absolute path

The current content of `.gitignore` is using relative paths. For
example, `test/config.log` would be ignored while it doesn't seem to be
the intent. Use absolute paths. They are still relative to the root of
the repository.

.gitignore

@@ -1,21 +1,21 @@
 *.o
-Makefile
+/Makefile
-Makefile.in
+/Makefile.in
-aclocal.m4
+/aclocal.m4
-autom4te.cache/
+/autom4te.cache/
-config.guess
+/config.guess
-config.log
+/config.log
-config.status
+/config.status
-config.sub
+/config.sub
-configure
+/configure
-depcomp
+/depcomp
-doc/Makefile
+/doc/Makefile
-doc/Makefile.in
+/doc/Makefile.in
-install-sh
+/install-sh
-missing
+/missing
-src/.deps/
+/src/.deps/
-src/Makefile
+/src/Makefile
-src/Makefile.in
+/src/Makefile.in
-src/s3fs
+/src/s3fs
-test/Makefile
+/test/Makefile
-test/Makefile.in
+/test/Makefile.in