make_absolute_path: return the input path if it points to our buffer
make_absolute_path: return the input path if it points to our buffer

Some codepaths call make_absolute_path with its own return value as
input. In such a cases, return the path immediately.

This fixes a valgrind-discovered error, whereby we tried to copy a
string onto itself.

Signed-off-by: Carlos Martín Nieto <eecb638c0e60875b4a70118790d5be43bb59c2d6@elego.de>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git