Optimize away git_text_gather_stats in diff

GProf shows `git_text_gather_stats` as the most expensive call
in large diffs.  The function calculates a lot of information
that is not actually used and does not do so in a optimal
order.  This introduces a tuned `git_buf_is_binary` function
that executes the same algorithm in a fraction of the time.

1 files changed, 18 insertions, 0 deletions

diff --git a/src/buffer.c b/src/buffer.c
index ef95839f6..29aaf3fec 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -445,3 +445,21 @@ int git_buf_common_prefix(git_buf *buf, const git_strarray *strings)
 
 	return 0;
 }
+
+bool git_buf_is_binary(const git_buf *buf)
+{
+	int i, printable = 0, nonprintable = 0;
+
+	for (i = 0; i < buf->size; i++) {
+		unsigned char c = buf->ptr[i];
+		if (c > 0x1F && c < 0x7f)
+			printable++;
+		else if (c == '\0')
+			return true;
+		else if (!git__isspace(c))
+			nonprintable++;
+	}
+
+	return ((printable >> 7) < nonprintable);
+}
+