RFR: cli: add new command hg-export [v2]

[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

Erik Helin has updated the pull request incrementally with one additional commit since the last revision:

  Fix spelling
  
  Co-authored-by: Robin Westberg <robin at westberg.se>

-------------

Changes:
  - all: https://git.openjdk.java.net/skara/pull/712/files
  - new: https://git.openjdk.java.net/skara/pull/712/files/2b501612..291f1ac5

Webrevs:
 - full: https://webrevs.openjdk.java.net/skara/712/webrev.01
 - incr: https://webrevs.openjdk.java.net/skara/712/webrev.00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 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