exec_cmd: rename to use dash in file name

This is more consistent with the project style. The majority of Git's
source files use dashes in preference to underscores in their file names.

Signed-off-by: Stefan Beller <sbeller@google.com>
diff --git a/builtin/hash-object.c b/builtin/hash-object.c
index 526da5c..a9a3a19 100644
--- a/builtin/hash-object.c
+++ b/builtin/hash-object.c
@@ -9,7 +9,7 @@
 #include "blob.h"
 #include "quote.h"
 #include "parse-options.h"
-#include "exec_cmd.h"
+#include "exec-cmd.h"
 
 /*
  * This is to create corrupt objects for debugging and as such it