Git-style Mercurial diffs, like normal Git diffs, contain symlink mode

information. We've hit issues in the past with the server applying

patches containing symlink modes.
Since a Git-style diff otherwise represents a symlink change as a

standard file change, with the content being the old/new target names,

we addressed this problem by stripping out the symlink mode and treating

it as a normal file when patching for display in the diff viewer.
This is by no means a perfect solution (that is in development, with

initial steps having been done in Review Board 4.0.5), but it addresses

the issue in Git.
This change brings that fix over to Mercurial when working with

Git-style diffs. The code in Git has been split out into a common

function that both can call, simplifying maintenance and allowing both

to benefit from any future bug fixes.

Unit tests pass.
Tested with a customer who hit this problem. Verified that they were able to

review symlinks in Mercurial.