add a -i (case-insensitive) option.  unfortunately this is not unicode
aware yet, but it's still useful.

Reference: /n/atom/patch/applied/diff-i
Date: Mon Sep 28 07:05:47 CES 2015
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/diff/diff.h	Mon Sep 28 07:04:30 2015
+++ /sys/src/cmd/diff/diff.h	Mon Sep 28 07:04:31 2015
@@ -2,6 +2,7 @@
 char bflag;			/* ignore multiple and trailing blanks */
 char rflag;			/* recurse down directory trees */
 char mflag;			/* pseudo flag: doing multiple files, one dir */
+char iflag;
 int anychange;
 extern Biobuf	stdout;
 extern int	binary;
--- /sys/src/cmd/diff/diffio.c	Mon Sep 28 07:04:33 2015
+++ /sys/src/cmd/diff/diffio.c	Mon Sep 28 07:04:34 2015
@@ -47,6 +47,14 @@
 	return p - buf;
 }
 
+static int
+caseadj(int c)
+{
+	if(iflag && c >= 'A' && c <= 'Z')
+		c += 0x20;
+	return c;
+}
+
 #define HALFLONG 16
 #define low(x)	(x&((1L<<HALFLONG)-1))
 #define high(x)	(x>>HALFLONG)
@@ -59,7 +67,7 @@
 static int
 readhash(Biobuf *bp, char *buf)
 {
-	long sum;
+	long sum, c;
 	unsigned shift;
 	char *p;
 	int len, space;
@@ -73,7 +81,7 @@
 	{
 	case 0:
 		while (len--) {
-			sum += (long)*p++ << (shift &= (HALFLONG-1));
+			sum += caseadj(*p++) << (shift &= (HALFLONG-1));
 			shift += 7;
 		}
 		break;
@@ -82,7 +90,8 @@
 		 * coalesce multiple white-space
 		 */
 		for (space = 0; len--; p++) {
-			if (isspace(*p)) {
+			c = caseadj(*p);
+			if (isspace(c)) {
 				space++;
 				continue;
 			}
@@ -90,7 +99,7 @@
 				shift += 7;
 				space = 0;
 			}
-			sum += (long)*p << (shift &= (HALFLONG-1));
+			sum += c << (shift &= (HALFLONG-1));
 			shift += 7;
 		}
 		break;
@@ -99,11 +108,10 @@
 		 * strip all white-space
 		 */
 		while (len--) {
-			if (isspace(*p)) {
-				p++;
+			c = caseadj(*p++);
+			if (isspace(c))
 				continue;
-			}
-			sum += (long)*p++ << (shift &= (HALFLONG-1));
+			sum += c << (shift &= (HALFLONG-1));
 			shift += 7;
 		}
 		break;
--- /sys/src/cmd/diff/diffreg.c	Mon Sep 28 07:04:36 2015
+++ /sys/src/cmd/diff/diffreg.c	Mon Sep 28 07:04:38 2015
@@ -316,7 +316,7 @@
 	flushchanges();
 }
 
-#define BUF 32768
+#define BUF 4096
 static int
 cmp(Biobuf* b1, Biobuf* b2)
 {
--- /sys/src/cmd/diff/main.c	Mon Sep 28 07:04:39 2015
+++ /sys/src/cmd/diff/main.c	Mon Sep 28 07:04:40 2015
@@ -11,7 +11,7 @@
 static char *tmp[] = {"/tmp/diff1XXXXXXXXXXX", "/tmp/diff2XXXXXXXXXXX"};
 static int whichtmp;
 static char *progname;
-static char usage[] = "diff [-abcefmnrw] file1 ... file2\n";
+static char usage[] = "diff [-abcefimnrw] file1 ... file2\n";
 
 static void
 rmtmpfiles(void)
@@ -209,6 +209,10 @@
 
 			case 'm':
 				mflag = 1;	
+				break;
+
+			case 'i':
+				iflag = 1;
 				break;
 
 			case 'h':
--- /sys/man/1/diff	Mon Sep 28 07:04:42 2015
+++ /sys/man/1/diff	Mon Sep 28 07:04:43 2015
@@ -4,7 +4,7 @@
 .SH SYNOPSIS
 .B diff
 [
-.B -abcefmnrw
+.B -abcefimnrw
 ] 
 .I file1 ... file2
 .SH DESCRIPTION
@@ -93,6 +93,13 @@
 .B -w
 option causes all white-space to be removed from input lines
 before applying the difference algorithm.
+The
+.B -i
+option causes case to be folded before applying the difference
+algorithm, and may be combined with either
+.B -b
+or
+.BR -w .
 .PP
 The
 .B -n
@@ -174,3 +181,7 @@
 .I diff
 on directories, the notion of what is a text
 file is open to debate.
+.PP
+The
+.B -i
+flag is not Unicode aware.