diff: avoid empty hunks when there are no changes

Currently, diff outputs a file header, even if there are
no changes to the file. This is wonky.

It means that the header chunks are ambiguous, since
not all header chunks are followed by '@@ hunk', and
'--- file', '+++ file' lines can be generated from
file content.

This changes the way that we decide to print the file
header, so we only print it when outputting the first
hunk on flushchanges.

Flushchanges is called once per regular file, at the
end of `diffreg`, so we output a hunk header once per
file.

3 files changed, 8 insertions, 13 deletions

diff --git a/sys/src/cmd/diff/diff.h b/sys/src/cmd/diff/diff.h
index 8aca0439f..e1c8263d6 100644
--- a/sys/src/cmd/diff/diff.h
+++ b/sys/src/cmd/diff/diff.h
@@ -28,6 +28,5 @@ Biobuf *prepare(int, char *, char *);
 void panic(int, char *, ...);
 void check(Biobuf *, Biobuf *);
 void change(int, int, int, int);
-void fileheader(void);
 void flushchanges(void);
 
diff --git a/sys/src/cmd/diff/diffio.c b/sys/src/cmd/diff/diffio.c
index b577b6d2a..1d4d4d6bb 100644
--- a/sys/src/cmd/diff/diffio.c
+++ b/sys/src/cmd/diff/diffio.c
@@ -330,23 +330,15 @@ changeset(int i)
 }
 
 void
-fileheader(void)
-{
-	if(mode != 'u')
-		return;
-	Bprint(&stdout, "--- %s\n", file1);
-	Bprint(&stdout, "+++ %s\n", file2);
-}
-
-void
 flushchanges(void)
 {
-	int a, b, c, d, at;
+	int a, b, c, d, at, hdr;
 	int i, j;
 
 	if(nchanges == 0)
 		return;
-	
+
+	hdr = 0;
 	for(i=0; i<nchanges; ){
 		j = changeset(i);
 		a = changes[i].a-Lines;
@@ -369,6 +361,11 @@ flushchanges(void)
 			j = nchanges;
 		}
 		if(mode == 'u'){
+			if(!hdr){
+				Bprint(&stdout, "--- %s\n", file1);
+				Bprint(&stdout, "+++ %s\n", file2);
+				hdr = 1;
+			}
 			Bprint(&stdout, "@@ -%d,%d +%d,%d @@\n", a, b-a+1, c, d-c+1);
 		}else{
 			Bprint(&stdout, "%s:", file1);
diff --git a/sys/src/cmd/diff/diffreg.c b/sys/src/cmd/diff/diffreg.c
index f773a4edf..e37ffa7a3 100644
--- a/sys/src/cmd/diff/diffreg.c
+++ b/sys/src/cmd/diff/diffreg.c
@@ -284,7 +284,6 @@ output(void)
 	m = len[0];
 	J[0] = 0;
 	J[m+1] = len[1]+1;
-	fileheader();
 	if (mode != 'e') {
 		for (i0 = 1; i0 <= m; i0 = i1+1) {
 			while (i0 <= m && J[i0] == J[i0-1]+1)