[PATCH] diff: trivial fix for --output file error message

[Matthieu Moy <Matthieu.Moy@imag.fr>]

Since we use parse_long_opt(), the option argument is either after
--output=... or in the next command-line argument. optarg is the reliable
way to access it.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
---

On my system, it seems --output file works, probably because the
argument array is stored as "--output\0file", but we should definitely
not rely on that ...

 diff.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/diff.c b/diff.c
index 276e029..38f5f9c 100644
--- a/diff.c
+++ b/diff.c
@@ -3315,7 +3315,7 @@ int diff_opt_parse(struct diff_options *options, const char **av, int ac)
 	else if ((argcount = parse_long_opt("output", av, &optarg))) {
 		options->file = fopen(optarg, "w");
 		if (!options->file)
-			die_errno("Could not open '%s'", arg + strlen("--output="));
+			die_errno("Could not open '%s'", optarg);
 		options->close_file = 1;
 		return argcount;
 	} else
-- 
1.7.3.2.g257b5f