am: ignore leading whitespace before patch

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>