RFR: cli: add new command hg-export

[Erik Helin]

Hi all,

please review this patch that adds the new command `git hg-export`. The `git hg-export` command prints information
about a Git commit in the format expected by `hg import` (i.e. it is very similar to the `hg export --git` command).
This command is primarily useful for backporting changes from a Git repository to a Mercurial repository as a
backporter can now run:

$ git clone https://git.openjdk.java.net/jdk
$ cd jdk
$ git hg-export <REV> | hg -R /path/to/hg/repo/for/jdk11u-dev import -

The command `git hg-export` will output the commit message in the format used by the OpenJDK Mercurial repositories.
The command will also output the correct author and date.

Testing:
- [x] Manual testing of `git hg-export` on Linux x64

Thanks,
Erik

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Commit messages:
 - Remove trailing whitespace
 - cli: new command hg-export

Changes: https://git.openjdk.java.net/skara/pull/712/files
 Webrev: https://webrevs.openjdk.java.net/skara/712/webrev.00
  Stats: 135 lines in 6 files changed: 131 ins; 3 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/712.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/712/head:pull/712

PR: https://git.openjdk.java.net/skara/pull/712