New command to export a patch and allow existing log command to take opt...

...ions.

* HgExportPatch command takes patch name as input. If the env variable
  HG_EXPORT_PATCH_DIR is set, then the patch will be created under it.
  Otherwise, it will be created in the directory from which vim
  was launched.

* HgLog command takes options that can be passed to hg log command.
  E.g., the following command will list just 3 logs by user bob.
  :Hglog -u bob -l 3

Testing:
* Patch gets created under the right directory when env variable is set
  and not set.

* Hglog command honors -u and -l options. It also works when current
  file name is given as input --> :Hglog % -u bob -l 3
I've been meaning to do screencasts for a while now... anyway I tend to do:

1. Get the status (at the bottom, almost like a status bar a few lines high if I only have a few modified files)
2. Select a file, and either do `<C-H>` or `<C-D>` (more on that later)
3. Switch back to the status window below, select a different file, etc.
4. Quit the status window. This should close any diff windows it opened.

If I want to truly look at the differences between 2 revisions, I'll do `<C-D>`, which opens a new tab with a full fledged Vim diff. Nothing fancy here.

However, if I just want to make sure things are tidy before committing, I'll do `<C-H>`, which should open a "diff summary" in a split window above (basically splitting the main file's window, above the status window, in 2 panes: one for the file, one for the diff summary). Then I start cleaning up any stray debug lines, any whitespace differences and other useless things, etc. The beauty here is that I can go to the right pane (the one with the diff summary) and go `:e`... it will refresh just like that and show me the updated diff summary. No need to get back to the status window and redo `<C-H>`.

But either way, I guess it would be nice to have options for people who prefer the status window on top, or have a status window that always takes 50% of the height.