When merging from branch `front` to `test` the following test case
fails:

<testcase>
  term% git/init merge.fail.git
  term% cd merge.fail.git
  term% mkdir A
  term% cp /sys/src/cmd/xargs.c A/
  term% git/add A/xargs.c
  term% git/commit -m 'Meh...' A/xargs.c
  heads/front: 5c36636e55aac3341ce42afee51b73106e8f1bc5
  term% git/branch -n test
  refs/heads/test: 5c36636e55aac3341ce42afee51b73106e8f1bc5
  term% mkdir X
  term% cp /sys/src/cmd/xargs.c X/
  term% git/add X/xargs.c
  term% git/commit -m 'Nein...' X/xargs.c
  heads/test: 6dc8fecb03f15e1c018c51306d25b10919e8fbe4
  term% git/branch front
  refs/heads/front: 5c36636e55aac3341ce42afee51b73106e8f1bc5
  term% mkdir A/B
  term% cp /sys/src/cmd/xargs.c A/B/
  term% git/add A/B/xargs.c
  term% git/commit -m 'NeinNein...' A/B/xargs.c
  heads/front: 3ca03f92152ed170e12b1ce2416ababbc18ffe54
  term% git/branch test
  refs/heads/test: 6dc8fecb03f15e1c018c51306d25b10919e8fbe4
  term% mkdir X/Y
  term% cp /sys/src/cmd/xargs.c X/Y/
  term% git/add X/Y/xargs.c
  term% git/commit -m  'NeinNeinNein...' X/Y/xargs.c
  heads/test: a81075176ae6dbd6f501e6ea43fd878a29b9c60d
  term% git/branch front
  refs/heads/front: 3ca03f92152ed170e12b1ce2416ababbc18ffe54
  term% git/branch test
  refs/heads/test: a81075176ae6dbd6f501e6ea43fd878a29b9c60d
  term% git/merge front
  /sys/lib/git/common.rc:88: > can't create: ./A/B/.tmp: './A/B' does not exist
</testcase>

The problem is a redirect in `/sys/lib/git/common.rc` that assumes the
directory component of the file that contains the diff exists:

<sys/lib/git/common.rc>
fn merge1 {@{
	rfork e
	n=$pid
	out=$1
	ours=$2
	base=$3
	theirs=$4
	tmp=$out.tmp
	while(test -f $tmp){
		tmp=$tmp.$n
		n=`{echo $n + 1 | hoc}
	}

	if(! test -f $ours)
		ours=/dev/null
	if(! test -f $base)
		base=/dev/null
	if(! test -f $theirs)
		theirs=/dev/null
	if(! ape/diff3 -3 -m $ours $base $theirs > $tmp)
		echo merge needed: $out >[1=2]
^^^^^^^^^^^^^^^^
IF THE FOLDER WHERE $tmp LIVES DOES NOT EXIST A REDIRECT
TO `> $tmp` FAILS TRIPPING UP THE MERGE

	if(mergeperm $ours $base $theirs){
		mv $tmp $out
		git/add $out
		chmod $mergedperms $out
	}
	if not {
		rm -f $tmp $out
		git/rm $out
	}
}}
</sys/lib/git/common.rc>

A solution is to check if the folder exists and create it via `mkdir -p`
in case it doesn't.

Can someone have a look and confirm this is the right solution and not
a hack. If this is Ok plz let me know and I can push the fix.

Thanks.

Cheers,
Igor

---
diff 45f713a2bc2961b3c920aed616188591612da210 3c2aa08560fba8a69e09731e3e5d72f7003021e3
--- a/sys/lib/git/common.rc	Sun Apr 17 23:25:04 2022
+++ b/sys/lib/git/common.rc	Tue Apr 19 13:19:00 2022
@@ -76,7 +76,8 @@
 		tmp=$tmp.$n
 		n=`{echo $n + 1 | hoc}
 	}
-
+	if(! test -d `{basename -d $tmp})
+		mkdir -p `{basename -d $tmp}
 	if(! test -f $ours)
 		ours=/dev/null
 	if(! test -f $base)