Add -e/--exclude to git-clean.

With the -e/--exclude option for git-clean, a user can specify files
that they haven't yet told git about, but either need for a short amount
of time or plan to tell git about them later. This allows one to still
use git-clean while these files are around without losing data.

Signed-off-by: Jared Hance <jaredhance@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-clean.txt

@@ -8,7 +8,7 @@ git-clean - Remove untracked files from the working tree
 SYNOPSIS
 --------
 [verse]
-'git clean' [-d] [-f] [-n] [-q] [-x | -X] [--] <path>...
+'git clean' [-d] [-f] [-n] [-q] [-e <pattern>] [-x | -X] [--] <path>...
 
 DESCRIPTION
 -----------
@@ -45,6 +45,12 @@ OPTIONS
 	Be quiet, only report errors, but not the files that are
 	successfully removed.
 
+-e <pattern>::
+--exclude=<pattern>::
+	Specify special exceptions to not be cleaned.  Each <pattern> is
+	the same form as in $GIT_DIR/info/excludes and this option can be
+	given multiple times.
+
 -x::
 	Don't use the ignore rules.  This allows removing all untracked
 	files, including build products.  This can be used (possibly in