git/{diff,import}: make it easier to handle manually-asembled patch emails

Often, people (including myself) will write emails that
can almost be applied with git/import.  This changes
git/diff and git/import so that things will generally
work even when assembling diffs by hand:

1.	git/import becomes slightly more lax:

		^diff ...
		^--- ...

	will both be detected as the start of a patch.

2.  git/diff produces the same format of diff
	as git/export, starting with paths:

		--- a/path/to/file
		+++	b/path/to/file

	which means that the 'ape/patch -p1' used
	within git/import will just work.

So with this, if you send an email to the mailing list,
write up a committable description, and append the
output of git/diff to the end of the email, git/import
should just work.

[this patch was send through the mailing list using the
above procedure, and will be committed with git/import
to verify that it works as advertised]

sys/src/cmd/git/diff

@@ -26,12 +26,16 @@ fn lsdirty {
 		git/query -c $commit HEAD | subst '^..'
 }
 
+mntgen /mnt/scratch
+bind $branch/tree/ /mnt/scratch/a
+bind . /mnt/scratch/b
+echo diff `{git/query $commit} uncommitted
 for(f in `$nl{lsdirty | sort | uniq}){
-	orig=$branch/tree/$f
-	if(! test -f $orig)
+	cd /mnt/scratch
+	if(! test -f a/$f)
 		orig=/dev/null
-	if(! test -f $f)
+	if(! test -f b/$f)
 		f=/dev/null
-	diff -u $orig $f
+	diff -u a/$f b/$f
 }
 exit ''

sys/src/cmd/git/import

@@ -45,7 +45,7 @@ fn apply @{
 		state="body"
 		next
 	}
-	(state=="headers" || state=="body") && (/^diff/ || /^---[ 	]*$/){
+	(state=="headers" || state=="body") && (/^diff / || /^--- /){
 		state="diff"
 	}
 	state=="body" {