bug? illegal text in commit log

["Michael S. Tsirkin" <mst@redhat.com>]

It seems that it's not too hard to make commits such that
the standard way of git format-patch -> email -> git am
fails.

For example:

[mst@tuck ~]$ mkdir gittest
[mst@tuck ~]$ cd gittest/
[mst@tuck gittest]$ git init
Initialized empty Git repository in /home/mst/gittest/.git/
[mst@tuck gittest]$ echo AA > a.txt
[mst@tuck gittest]$ git add a.txt
[mst@tuck gittest]$ git commit
[master (root-commit) 46e2cd1] original
 1 file changed, 1 insertion(+)
 create mode 100644 a.txt
[mst@tuck gittest]$ echo AA > b.txt
[mst@tuck gittest]$ echo BB > c.txt
[mst@tuck gittest]$ diff -u b.txt c.txt > bc.diff
[mst@tuck gittest]$ echo "this change" > commitlog.txt
[mst@tuck gittest]$ echo >> commitlog.txt
[mst@tuck gittest]$ cat bc.diff >> commitlog.txt
[mst@tuck gittest]$ echo BB >> a.txt
[mst@tuck gittest]$ git commit -a -F commitlog.txt 
[master c5f40b2] this change
 1 file changed, 1 insertion(+)
[mst@tuck gittest]$ git format-patch HEAD~1..
0001-this-change.patch
[mst@tuck gittest]$ git reset --hard HEAD~1
HEAD is now at 46e2cd1 original
[mst@tuck gittest]$ git am 0001-this-change.patch
Applying: this change
error: sha1 information is lacking or useless (c.txt).
error: could not build fake ancestor
Patch failed at 0001 this change
hint: Use 'git am --show-current-patch' to see the failed patch
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".

A work around is "don't do it" - avoid putting anything that looks like
a unified diff in the commit log.

However:
- Users don't know what other restrictions exist
- User sending the patch has no way to detect failure,
  it's only visible to user receiving the patch

Ideas:
- validate commit log and warn users?
- find a way to escape text in git format-patch, and unescape in git am?


Thanks,
-- 
MST