When "-" is used for standard input, the filename in the context/unified headers should be "-", not "/dev/stdin"

[oSoMoN]

When passing in "-" as a parameter on the CLI invokation to use the standard input for either of the input files, in conjunction with the -c or -u options, the filenames displayed should be "-", not "/dev/stdin", to be consistent with GNU's implementation:

echo foo > foo

echo bar | /usr/bin/diff -u foo -
--- foo	2024-04-03 18:55:48.837847038 +0200
+++ -	2024-04-03 18:58:47.031319515 +0200
@@ -1 +1 @@
-foo
+bar

echo bar | cargo run --release -- -u foo -
--- foo	
+++ /dev/stdin	
@@ -1 +1 @@
-foo
+bar