manual: Fix example finding commits referencing given content.

manual: Fix example finding commits referencing given content.

If I'm handed a file, then it typically lives outside the
working directory.  git-log only operates on in-tree files,
so the first 'filename' should be an in-tree one, or it should
look at all files.  This patch does the latter, so it would
also find renamed files.  However, it is also slower.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>