git-am: cope better with an empty Subject: line

When the Subject: line is empty for whatever reason, git-am was fooled by
it and left an empty line at the beginning of the resulting commit log
message.

This moves the logic around so that we do not keep $SUBJECT in a separate
variable.  Instead, $dotest/msg-clean, which used to be the log message
body extracted from the message and then trailing whitespaces cleansed
out, now contains the subject line followed by a blank line at the
beginning for normal messages, and we use the first line from the file as
the summary line throughout the program.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-am.sh

@@ -107,7 +107,7 @@ It does not apply to blobs recorded in its index."
     # patch did not touch, so recursive ends up canceling them,
     # saying that we reverted all those changes.
 
-    eval GITHEAD_$his_tree='"$SUBJECT"'
+    eval GITHEAD_$his_tree='"$FIRSTLINE"'
     export GITHEAD_$his_tree
     git-merge-recursive $orig_tree -- HEAD $his_tree || {
 	    git rerere
@@ -117,10 +117,6 @@ It does not apply to blobs recorded in its index."
     unset GITHEAD_$his_tree
 }
 
-reread_subject () {
-	git stripspace <"$1" | sed -e 1q
-}
-
 prec=4
 dotest=.dotest sign= utf8=t keep= skip= interactive= resolved= binary=
 resolvemsg= resume=
@@ -303,7 +299,11 @@ do
 			echo "Patch is empty.  Was it split wrong?"
 			stop_here $this
 		}
-		git stripspace < "$dotest/msg" > "$dotest/msg-clean"
+		SUBJECT="$(sed -n '/^Subject/ s/Subject: //p' "$dotest/info")"
+		case "$keep_subject" in -k)  SUBJECT="[PATCH] $SUBJECT" ;; esac
+
+		(echo "$SUBJECT" ; echo ; cat "$dotest/msg") |
+			git stripspace > "$dotest/msg-clean"
 		;;
 	esac
 
@@ -319,9 +319,6 @@ do
 
 	export GIT_AUTHOR_NAME GIT_AUTHOR_EMAIL GIT_AUTHOR_DATE
 
-	SUBJECT="$(sed -n '/^Subject/ s/Subject: //p' "$dotest/info")"
-	case "$keep_subject" in -k)  SUBJECT="[PATCH] $SUBJECT" ;; esac
-
 	case "$resume" in
 	'')
 	    if test '' != "$SIGNOFF"
@@ -340,10 +337,8 @@ do
 		ADD_SIGNOFF=
 	    fi
 	    {
-		printf '%s\n' "$SUBJECT"
 		if test -s "$dotest/msg-clean"
 		then
-			echo
 			cat "$dotest/msg-clean"
 		fi
 		if test '' != "$ADD_SIGNOFF"
@@ -360,6 +355,7 @@ do
 			;;
 		esac
 	esac
+	FIRSTLINE=$(head -1 "$dotest/final-commit")
 
 	resume=
 	if test "$interactive" = t
@@ -380,7 +376,7 @@ do
 		[aA]*) action=yes interactive= ;;
 		[nN]*) action=skip ;;
 		[eE]*) git_editor "$dotest/final-commit"
-		       SUBJECT=$(reread_subject "$dotest/final-commit")
+		       FIRSTLINE=$(head -1 "$dotest/final-commit")
 		       action=again ;;
 		[vV]*) action=again
 		       LESS=-S ${PAGER:-less} "$dotest/patch" ;;
@@ -403,7 +399,7 @@ do
 		stop_here $this
 	fi
 
-	printf 'Applying %s\n' "$SUBJECT"
+	printf 'Applying %s\n' "$FIRSTLINE"
 
 	case "$resolved" in
 	'')
@@ -461,7 +457,7 @@ do
 	tree=$(git write-tree) &&
 	parent=$(git rev-parse --verify HEAD) &&
 	commit=$(git commit-tree $tree -p $parent <"$dotest/final-commit") &&
-	git update-ref -m "$GIT_REFLOG_ACTION: $SUBJECT" HEAD $commit $parent ||
+	git update-ref -m "$GIT_REFLOG_ACTION: $FIRSTLINE" HEAD $commit $parent ||
 	stop_here $this
 
 	if test -x "$GIT_DIR"/hooks/post-applypatch