git-config: handle --file option with relative pathname properly

git-config: handle --file option with relative pathname properly

When calling git-config not from the top level directory of a repository,
it changes directory before trying to open the config file specified
through the --file option, which then fails if the config file was
specified by a relative pathname.  This patch adjusts the pathname to
the config file if applicable.

The problem was noticed by Joey Hess, reported through
 http://bugs.debian.org/445208

Signed-off-by: Gerrit Pape <pape@smarden.org>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>