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>

http-backend.c

@@ -5,7 +5,7 @@
 #include "pkt-line.h"
 #include "object.h"
 #include "tag.h"
-#include "exec_cmd.h"
+#include "exec-cmd.h"
 #include "run-command.h"
 #include "string-list.h"
 #include "url.h"