Replace top-level .gitignore with .gitignore_global, and add .hgignor…

…e_global

The rationale for this name switch is as follows:

0. These two files will be automatically kept up-to-date with
   svn:ignore properties in the tree via a cron script.  Meaning: if a
   developer makes changes to svn:ignore properties, those changes
   will automatically be reflected in .gitignore_global /
   .hgignore_global within a fixed time period (at the moment,
   nightly).

1. Developers who make pure clones of git/hg OMPI repos can copy the
   relevant .*ignore_global file to .*ignore.  This gives them a good
   starting point for a fully-populated ignore file.  However, since
   the upstream _global file may change over time, developers will
   likely want to keep them in sync with their local .*ignore file.
   Here's two possibilities:

   1a. Maintain a single .*ignore file and manually merge in changes
       from the _global file upstream.

   1b. Maintain their local ignores in a .*ignore_local file, and
       periodically "cat .*ignore_global .*ignore_local > *.ignore"
       (e.g., whenever the upstream _global file changes).

2. Developers who make svn+clone checkouts can do similar things as
   listed in 1, with the added feature that they cannot make the
   mistake of committing their locally-modified .*ignore file back to
   SVN (presumably containing references to files that do not exist in
   SVN), since the .*ignore file is not in SVN.

Point #2 is probably the    stronger of these two reasons -- prevent
mistakes of developers accidentally committing locally-modified
.*ignore files back to SVN.

This commit was SVN r31408.