fix --ln-untracked with relative directories

We used to take files from the initial directory, which may or may not be
the same as the directory containing the main file (e.g. --main
foo/bar.tex), and link them to the directory corresponding to the
checkout of the main file.

Fix this, and always link files from the directory containing the main file.

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 (must be in current directory!)
+    --main <file.tex>     name of the main LaTeX file
     --no-view             don't display the resulting PDF file
     --bibtex, -b          run bibtex as well as latex
                              (pdflatex,bibtex,pdflatex,pdflatex)
@@ -144,6 +144,7 @@ bibtex=0
 biber=0
 output=
 initial_dir=$PWD
+initial_repo=$(git rev-parse --show-toplevel)
 tmpdir_prefix="/tmp"
 prepare_cmd=
 flatten=1
@@ -420,7 +421,7 @@ verbose "Checking out old and new version"
 cd old || die "Cannot cd to old/"
 if test "$ln_untracked" = 1; then
     (
-	mkdir -p "$maindir" && cd "$maindir" && ln -s "$initial_dir"/* .
+       mkdir -p "$maindir" && cd "$maindir" && ln -s "$initial_repo"/"$maindir"/* .
     )
 fi
 
@@ -436,7 +437,7 @@ verbose_progress
 cd ../new || die "Cannot cd to new/"
 if test "$ln_untracked" = 1; then
     (
-	mkdir -p "$maindir" && cd "$maindir" && ln -s "$initial_dir"/* .
+       mkdir -p "$maindir" && cd "$maindir" && ln -s "$initial_repo"/"$maindir"/* .
     )
 fi
 if test "$new" != "--"; then