emacs: Observe the charset of MIME parts when reading them.

`notmuch--get-bodypart-raw' previously assumed that all non-binary MIME
parts could be successfully read by assuming that they were UTF-8
encoded. This was demonstrated to be wrong, specifically when a part was
marked as ISO8859-1 and included accented characters (which were
incorrectly rendered as a result).

Rather than assuming UTF-8, attempt to use the part's declared charset
when reading it, falling back to US-ASCII if the declared charset is
unknown, unsupported or invalid.

emacs/notmuch-lib.el

@@ -23,6 +23,7 @@
 
 ;;; Code:
 
+(require 'mm-util)
 (require 'mm-view)
 (require 'mm-decode)
 (require 'cl)
@@ -572,7 +573,20 @@ the given type."
 				   ,@(when process-crypto '("--decrypt"))
 				   ,(notmuch-id-to-query (plist-get msg :id))))
 			   (coding-system-for-read
-			    (if binaryp 'no-conversion 'utf-8)))
+			    (if binaryp 'no-conversion
+			      (let ((coding-system (mm-charset-to-coding-system
+						    (plist-get part :content-charset))))
+				;; Sadly,
+				;; `mm-charset-to-coding-system' seems
+				;; to return things that are not
+				;; considered acceptable values for
+				;; `coding-system-for-read'.
+				(if (coding-system-p coding-system)
+				    coding-system
+				  ;; RFC 2047 says that the default
+				  ;; charset is US-ASCII. RFC6657
+				  ;; complicates this somewhat.
+				  'us-ascii)))))
 		       (apply #'call-process notmuch-command nil '(t nil) nil args)
 		       (buffer-string))))))
     (when (and cache data)