Merge branch 'cn/bom-in-gitignore'

Teach the codepaths that read .gitignore and .gitattributes files
that these files encoded in UTF-8 may have UTF-8 BOM marker at the
beginning; this makes it in line with what we do for configuration
files already.

* cn/bom-in-gitignore:
  attr: skip UTF8 BOM at the beginning of the input file
  config: use utf8_bom[] from utf.[ch] in git_parse_source()
  utf8-bom: introduce skip_utf8_bom() helper
  add_excludes_from_file: clarify the bom skipping logic
  dir: allow a BOM at the beginning of exclude files

attr.c

@@ -12,6 +12,7 @@
 #include "exec_cmd.h"
 #include "attr.h"
 #include "dir.h"
+#include "utf8.h"
 
 const char git_attr__true[] = "(builtin)true";
 const char git_attr__false[] = "\0(builtin)false";
@@ -379,8 +380,12 @@ static struct attr_stack *read_attr_from_file(const char *path, int macro_ok)
 		return NULL;
 	}
 	res = xcalloc(1, sizeof(*res));
-	while (fgets(buf, sizeof(buf), fp))
-		handle_attr_line(res, buf, path, ++lineno, macro_ok);
+	while (fgets(buf, sizeof(buf), fp)) {
+		char *bufp = buf;
+		if (!lineno)
+			skip_utf8_bom(&bufp, strlen(bufp));
+		handle_attr_line(res, bufp, path, ++lineno, macro_ok);
+	}
 	fclose(fp);
 	return res;
 }