am: ignore leading whitespace before patch

Some web-based email clients prepend whitespace to raw message
transcripts to workaround content-sniffing in some browsers.  Adjust
the patch format detection logic to ignore leading whitespace.

So now you can apply patches from GMail with "git am" in three steps:

 1. choose "show original"
 2. tell the browser to "save as" (for example by pressing Ctrl+S)
 3. run "git am" on the saved file

This fixes a regression introduced by v1.6.4-rc0~15^2~2 (git-am
foreign patch support: autodetect some patch formats, 2009-05-27).
GMail support was first introduced to "git am" by v1.5.4-rc0~274^2
(Make mailsplit and mailinfo strip whitespace from the start of the
input, 2007-11-01).

Signed-off-by: David Barr <davidbarr@google.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-am.sh

@@ -196,10 +196,15 @@ check_patch_format () {
 		return 0
 	fi
 
-	# otherwise, check the first few lines of the first patch to try
-	# to detect its format
+	# otherwise, check the first few non-blank lines of the first
+	# patch to try to detect its format
 	{
-		read l1
+		# Start from first line containing non-whitespace
+		l1=
+		while test -z "$l1"
+		do
+			read l1
+		done
 		read l2
 		read l3
 		case "$l1" in

t/t4150-am.sh

@@ -96,6 +96,13 @@ test_expect_success setup '
 		echo "X-Fake-Field: Line Three" &&
 		git format-patch --stdout first | sed -e "1d"
 	} | append_cr >patch1-crlf.eml &&
+	{
+		printf "%255s\\n" ""
+		echo "X-Fake-Field: Line One" &&
+		echo "X-Fake-Field: Line Two" &&
+		echo "X-Fake-Field: Line Three" &&
+		git format-patch --stdout first | sed -e "1d"
+	} > patch1-ws.eml &&
 
 	sed -n -e "3,\$p" msg >file &&
 	git add file &&
@@ -167,6 +174,17 @@ test_expect_success 'am applies patch e-mail not in a mbox with CRLF' '
 	test "$(git rev-parse second^)" = "$(git rev-parse HEAD^)"
 '
 
+test_expect_success 'am applies patch e-mail with preceding whitespace' '
+	rm -fr .git/rebase-apply &&
+	git reset --hard &&
+	git checkout first &&
+	git am patch1-ws.eml &&
+	! test -d .git/rebase-apply &&
+	git diff --exit-code second &&
+	test "$(git rev-parse second)" = "$(git rev-parse HEAD)" &&
+	test "$(git rev-parse second^)" = "$(git rev-parse HEAD^)"
+'
+
 test_expect_success 'setup: new author and committer' '
 	GIT_AUTHOR_NAME="Another Thor" &&
 	GIT_AUTHOR_EMAIL="a.thor@example.com" &&