Correct common spelling mistakes in comments and tests

Most of these were found using Lucas De Marchi's codespell tool.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Acked-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/xdiff/xhistogram.c b/xdiff/xhistogram.c
index bf99787..73210cb 100644
--- a/xdiff/xhistogram.c
+++ b/xdiff/xhistogram.c
@@ -55,7 +55,7 @@
 	struct record {
 		unsigned int ptr, cnt;
 		struct record *next;
-	} **records, /* an ocurrence */
+	} **records, /* an occurrence */
 	  **line_map; /* map of line to record chain */
 	chastore_t rcha;
 	unsigned int *next_ptrs;