Fix problem with linking uncommited files

$git_prefix is now correctly prepended to $main. This is important for
recreating the original directory structure in the temporary directory
when linking uncommited files.

git-latexdiff

@@ -55,7 +55,7 @@ If "--" is used for NEW, then diff against the working directory.
 
 Options:
     --help                this help message
-    --main <file.tex>     name of the main LaTeX file
+    --main <file.tex>     name of the main LaTeX file (must be in current directory!)
     --no-view             don't display the resulting PDF file
     --bibtex, -b          run bibtex as well as latex
                              (pdflatex,bibtex,pdflatex,pdflatex)
@@ -363,6 +363,7 @@ log_cmd () {
     fi
 }
 
+# main is relative to initial_dir (if returned)
 if test -z "$main" ; then
     printf "%s" "No --main provided, trying to guess ... "
     main=$(git grep -l '^[ \t]*\\documentclass')
@@ -398,7 +399,10 @@ git_dir="$(git rev-parse --git-dir)" || die "Not a git repository?"
 cd "$(git rev-parse --show-cdup)" || die "Can't cd back to repository root"
 git_dir=$(cd "$git_dir"; pwd)
 
-main=$git_prefix/$main
+# make main relative to git root directory
+if test -n "$git_prefix" ; then
+    main=$git_prefix$main
+fi
 mainbase=$(basename "$main" .tex)
 maindir=$(dirname "$main")