Remove duplicated words split across lines in comments

This has been detected using some interesting tricks with sed, and the
method used is mentioned in details in the discussion below.

Author: Justin Pryzby
Discussion: https://postgr.es/m/20180908013109.GB15350@telsasoft.com

src/backend/access/gin/ginbtree.c

@@ -211,7 +211,7 @@ freeGinBtreeStack(GinBtreeStack *stack)
 /*
  * Try to find parent for current stack position. Returns correct parent and
  * child's offset in stack->parent. The root page is never released, to
- * to prevent conflict with vacuum process.
+ * prevent conflict with vacuum process.
  */
 static void
 ginFindParents(GinBtree btree, GinBtreeStack *stack)

src/backend/access/transam/twophase.c

@@ -1726,7 +1726,7 @@ RecreateTwoPhaseFile(TransactionId xid, void *content, int len)
  * possible that GXACTs that were valid at checkpoint start will no longer
  * exist if we wait a little bit. With typical checkpoint settings this
  * will be about 3 minutes for an online checkpoint, so as a result we
- * we expect that there will be no GXACTs that need to be copied to disk.
+ * expect that there will be no GXACTs that need to be copied to disk.
  *
  * If a GXACT remains valid across multiple checkpoints, it will already
  * be on disk so we don't bother to repeat that write.

src/backend/access/transam/xlogarchive.c

@@ -422,7 +422,7 @@ ExecuteRecoveryCommand(const char *command, const char *commandName, bool failOn
 /*
  * A file was restored from the archive under a temporary filename (path),
  * and now we want to keep it. Rename it under the permanent filename in
- * in pg_wal (xlogfname), replacing any existing file with the same name.
+ * pg_wal (xlogfname), replacing any existing file with the same name.
  */
 void
 KeepFileRestoredFromArchive(const char *path, const char *xlogfname)