git-am: make it easier after fixing up an unapplicable patch.

Instead of having the user to edit the mail message, let the hand merge
result stored in .dotest/patch and continue, which is easier to manage.

Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git-am.txt

@@ -58,10 +58,10 @@ recover from this in one of two ways:
 
 . hand resolve the conflict in the working directory, run 'git
   diff HEAD' to extract the merge result into a patch form and
-  replacing the patch part of the message in .dotest directory.
-  After doing this, run `git-reset --hard HEAD` to bring the
-  working tree to the state before half-applying the patch, then
-  re-run the command without any options.
+  replacing the patch in .dotest/patch file.  After doing this,
+  run `git-reset --hard HEAD` to bring the working tree to the
+  state before half-applying the patch, then re-run the command
+  without any options.
 
 The command refuses to process new mailboxes while `.dotest`
 directory exists, so if you decide to start over from scratch,

git-am.sh

@@ -153,6 +153,7 @@ if test -d "$dotest"
 then
 	test ",$#," = ",0," ||
 	die "previous dotest directory $dotest still exists but mbox given."
+	resume=yes
 else
 	# Make sure we are not given --skip
 	test ",$skip," = ,, ||
@@ -215,10 +216,15 @@ do
 		go_next
 		continue
 	}
+	case "$resume" in
+	'')
 		git-mailinfo $keep $utf8 "$dotest/msg" "$dotest/patch" \
 			<"$dotest/$msgnum" >"$dotest/info" ||
 			stop_here $this
 		git-stripspace < "$dotest/msg" > "$dotest/msg-clean"
+		;;
+	esac
+	resume=
 
 	GIT_AUTHOR_NAME="$(sed -n '/^Author/ s/Author: //p' "$dotest/info")"
 	GIT_AUTHOR_EMAIL="$(sed -n '/^Email/ s/Email: //p' "$dotest/info")"