doc: make links for hg role
The system has a bit of smartness:
:hg:`update` -> links to hg.1.html#update
:hg:`help diffs` -> links to hg.1.html#diffs
#!/bin/sh
"$TESTDIR/hghave" symlink || exit 80
rm -rf a
hg init a
cd a
ln -s foo link
hg add link
hg ci -mbad link
hg rm link
hg ci -mok
hg diff -g -r 0:1 > bad.patch
hg up 0
hg import --no-commit bad.patch
hg st