*: fix typos which duplicate a word

Fix typos in code comments which repeat various words.  Most of the
cases are simple in that they repeat a word that usually cannot be
repeated in a grammatically correct sentence.  Just remove the
incorrectly duplicated word in these cases and rewrap text, if needed.

A tricky case is usage of "that that", which is sometimes grammatically
correct.  However, an instance of this in "t7527-builtin-fsmonitor.sh"
doesn't need two words "that", because there is only one daemon being
discussed, so replace the second "that" with "the".

Reword code comment "entries exist on on-disk index" in function
update_one in file cache-tree.c, by replacing incorrect preposition "on"
with "in".

Signed-off-by: Andrei Rybak <rybak.a.v@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

run-command.c

@@ -1853,7 +1853,7 @@ enum start_bg_result start_bg_command(struct child_process *cmd,
 		 *
 		 * We also assume that `start_command()` does not add
 		 * us to the cleanup list.  And that it calls
-		 * calls `child_process_clear()`.
+		 * `child_process_clear()`.
 		 */
 		sbgr = SBGR_ERROR;
 		goto done;