merge-file: correctly find files when called in subdir

merge-file: correctly find files when called in subdir

Since b541248 (merge.conflictstyle: choose between "merge" and "diff3
-m" styles, 2008-08-29), git-merge-file uses setup_directory_gently(),
thus cd'ing around to find any possible config files to use.

This broke merge-file when it is called from within a subdirectory of
a repository, and the arguments are all relative paths.

Fix by prepending the prefix, as passed down from the main git
setup code, if there is any.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>