contrib/subtree should ship with manpage

[GoogleCodeExporter]

What steps will reproduce the problem?
1. Build git from source.
2. Download git-manpages-*.
3. Install both.
4. Attempt to build and install contrib/subtree.

What is the expected output? What do you see instead?
Expected: git-subtree goes into $PREFIX/libexec/, and git-subtree.1 goes into 
$PREFIX/share/man/man1/.
Actual: git-subtree.1 fails to be generated because my system doesn't ship 
asciidoc and xmlto, along with the better part of a dozen dependencies, by 
default.

What version of the product are you using? On what operating system?
v1.7.12.2 and v1.8.0.rc0.18.gf84667d, on OS X 10.8.2.

Please provide any additional information below.
Prototype patch attached.

Original issue reported on code.google.com by kngsp...@gmail.com on 6 Oct 2012 at 7:57

Attachments:

• subtree_manpage.patch