correct a few doubled-word nits in comments and documentation

Found by running this command:
$ git ls-files -z|xargs -0 perl -0777 -n \
 -e 'while (/\b(then?|[iao]n|i[fst]|but|f?or|at|and|[dt]o)\s+\1\b/gims)' \
 -e '  {' \
 -e '    $n = ($` =~ tr/\n/\n/ + 1);' \
 -e '    ($v = $&) =~ s/\n/\\n/g;' \
 -e '    print "$ARGV:$n:$v\n";' \
 -e '  }'

Why not just git grep -E ...?
That wouldn't work then the doubled words are separated by a newline.
This is derived from a Makefile syntax-check rule in gnulib's maint.mk:
  http://git.sv.gnu.org/cgit/gnulib.git/tree/top/maint.mkSigned-off-by: NJim Meyering <meyering@redhat.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-sh-i18n--envsubst.txt

@@ -25,7 +25,7 @@ plumbing scripts and/or are writing new ones.
 'git sh-i18n{litdd}envsubst' is Git's stripped-down copy of the GNU
 `envsubst(1)` program that comes with the GNU gettext package. It's
 used internally by linkgit:git-sh-i18n[1] to interpolate the variables
-passed to the the `eval_gettext` function.
+passed to the `eval_gettext` function.
 
 No promises are made about the interface, or that this
 program won't disappear without warning in the next version

attr.h

@@ -40,7 +40,7 @@ int git_check_attr(const char *path, int, struct git_attr_check *);
 
 /*
  * Retrieve all attributes that apply to the specified path.  *num
- * will be set the the number of attributes on the path; **check will
+ * will be set to the number of attributes on the path; **check will
  * be set to point at a newly-allocated array of git_attr_check
  * objects describing the attributes and their values.  *check must be
  * free()ed by the caller.

gitweb/static/js/lib/cookies.js

@@ -30,7 +30,7 @@
  *                            If a negative value is specified or a date in the past),
  *                            the cookie will be deleted.
  *                            If set to null or omitted, the cookie will be a session cookie
- *                            and will not be retained when the the browser exits.
+ *                            and will not be retained when the browser exits.
  * @param {String} [options.path] Restrict access of a cookie to particular directory
  *                               (default: path of page that created the cookie).
  * @param {String} [options.domain] Override what web sites are allowed to access cookie

po/README

@@ -178,7 +178,7 @@ used:
 
 C:
 
- - Include builtin.h at the top, it'll pull in in gettext.h, which
+ - Include builtin.h at the top, it'll pull in gettext.h, which
    defines the gettext interface. Consult with the list if you need to
    use gettext.h directly.