revamp the --bibtex and --biber options

--bibtex and --biber were just running bibtex and biber on the
resulting diff, but this is rarely what users want. What --bbl does is
much more sensible: it generates *.bbl files on old and new versions,
and use these *.bbl to compute the diff. No run of bibtex or biber is
needed on the diff, since the *.bbl are latexpand-ed into the *.tex
source file.

Change the meaning of --bibtex and --biber to do what --bbl used to
do. --bbl is now an alias for --bibtex, and --biber does the same as
--bbl, but using biber to generate the *.bbl file.

Old, essentially broken options are kept with different names
--run-bibtex and --run-biber, but I suspect to one is really using
them. We break backward compatibility, but the benefit of having short
and sweet option names for things that work should be higher than the
cost of backward compatibility.

git-latexdiff

@@ -93,10 +93,18 @@ Options:
     --no-view             don't display the resulting PDF file
     --latex               run latex instead of pdflatex
     --xelatex             run xelatex instead of pdflatex
-    --bibtex, -b          run bibtex as well as latex
+    --bibtex, --bbl       display changes in the bibliography
+                             (runs bibtex to generate *.bbl files and
+                             include them in the source file using
+                             latexpand --expand-bbl before computing
+                             the diff)
+    --biber               like --bibtex, but runs biber instead.
+    --run-bibtex, -b      run bibtex as well as latex to generate the PDF file
                              (pdflatex,bibtex,pdflatex,pdflatex)
-    --biber               run BibLaTex-Biber as well as latex
+                          NOTE: --bibtex usually works better
+    --run-biber           run BibLaTex-Biber as well as latex to generate the PDF file
                              (pdflatex,biber,pdflatex,pdflatex)
+                          NOTE: --biber usually works better
     --view                view the resulting PDF file
                             (default if -o is not used)
     --pdf-viewer <cmd>    use <cmd> to view the PDF file (default: \$PDFVIEWER)
@@ -137,7 +145,6 @@ Options:
                           a PDF file is produced
     --ignore-makefile     ignore the Makefile, build as though it doesn't exist
     -*                    other options are passed directly to latexdiff
-    --bbl                 shortcut to flatten a bbl file of the same name as the project
     --latexpand OPT       pass option OPT to latexpand. Use multiple times like
                           --latexpand OPT1 --latexpand OPT2 to pass multiple options.
     --latexdiff-flatten   use --flatten from latexdiff instead of latexpand
@@ -285,7 +292,7 @@ while test $# -ne 0; do
 	    cleanup=all
 	    ;;
 	"-b"|"--bibtex")
-	    bibtex=1
+	    bbl=1
 	    ;;
 	"--latex")
 	    latex=1
@@ -294,6 +301,13 @@ while test $# -ne 0; do
             xelatex=1
             ;;
 	"--biber")
+	    BIBTEX_EXEC=biber
+	    bbl=1
+	    ;;
+	"--run-bibtex")
+	    bibtex=1
+	    ;;
+	"--run-biber")
 	    biber=1
 	    ;;	
 	"--verbose"|"-v")