Documentation: clarify "git clean -e <pattern>"

The current explanation of -e can be misread as allowing the user to say

    I know 'git clean -XYZ' (substitute -XYZ with any option and/or
    parameter) will remove paths A, B, and C, and I want them all removed
    except for paths matching this pattern by adding '-e C' to the same
    command line, i.e. 'git clean -e C -XYZ'.

But that is not what this option does. It augments the set of ignore rules
from the command line, just like the same "-e <pattern>" argument does
with the "ls-files" command (the user could probably pass "-e \!C" to tell
the command to clean everything the command would normally remove, except
for C). Also error out when both -x and -e are given with an explanation of
what -e means---it is a symptom of misunderstanding what -e does.

It also fixes small style nit in the parameter to add_exclude() call. The
current code only works because EXC_CMDL happens to be defined as 0.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-clean.txt

@@ -47,12 +47,14 @@ OPTIONS
 
 -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.
+	In addition to those found in .gitignore (per directory) and
+	$GIT_DIR/info/exclude, also consider these patterns to be in the
+	set of the ignore rules in effect.
 
 -x::
-	Don't use the ignore rules.  This allows removing all untracked
+	Don't use the standard ignore rules read from .gitignore (per
+	directory) and $GIT_DIR/info/exclude, but do still use the ignore
+	rules given with `-e` options.  This allows removing all untracked
 	files, including build products.  This can be used (possibly in
 	conjunction with 'git reset') to create a pristine
 	working directory to test a clean build.

builtin/clean.c

@@ -54,7 +54,7 @@ int cmd_clean(int argc, const char **argv, const char *prefix)
 		OPT_BOOLEAN('d', NULL, &remove_directories,
 				"remove whole directories"),
 		{ OPTION_CALLBACK, 'e', "exclude", &exclude_list, "pattern",
-		  "exclude <pattern>", PARSE_OPT_NONEG, exclude_cb },
+		  "add <pattern> to ignore rules", PARSE_OPT_NONEG, exclude_cb },
 		OPT_BOOLEAN('x', NULL, &ignored, "remove ignored files, too"),
 		OPT_BOOLEAN('X', NULL, &ignored_only,
 				"remove only ignored files"),
@@ -98,7 +98,8 @@ int cmd_clean(int argc, const char **argv, const char *prefix)
 		setup_standard_excludes(&dir);
 
 	for (i = 0; i < exclude_list.nr; i++)
-		add_exclude(exclude_list.items[i].string, "", 0, dir.exclude_list);
+		add_exclude(exclude_list.items[i].string, "", 0,
+			    &dir.exclude_list[EXC_CMDL]);
 
 	pathspec = get_pathspec(prefix, argv);