git-changelog: option to only use merges

[jankatins]

using -m or --merges-only will only show the merge
commit messages. It will also use the body of the merge
commit message in addition to the subject, as github merges
usually have a useless subject line for commit messages.

If used with -n/--no-merges, the last one specified will win.

Will need regenerated man pages (.html + .1)

Closes: #470

[jankatins]

A output of this:

n.n.n / 2015-10-18
==================

  * Merge pull request #466 from anarcat/master
    add simple sed command
  * Merge pull request #465 from spacewander/fix-color
    Fix color
  * Merge pull request #460 from apjanke/summary-defensive-locale
    git-summary: protect against character encoding issues with
    LC_ALL=C
  * Merge pull request #458 from apjanke/makefile-refactor-libs
    Makefile: refactor is-git-repo inclusion logic
  * Merge pull request #459 from apjanke/effort-portable-colors
    git-effort: use portable terminal escape sequences
  * Merge pull request #454 from apjanke/makefile-escape-mktemp
    Makefile: escape $ used inside eval as shell variable
  * Merge pull request #453 from jhnns/pr-url
    Add possibility to also checkout pull requests based on
    GitHub urls
  * Merge pull request #452 from akimd/summary-locale
    summary: beware of locale issues, and pass options to
    line-summary
  * Merge pull request #443 from Natim/patch-1
    Make sure git-authors doesn't return twice the same ones.
[...]

[hemanth]

LGTM //cc @qw3rtman

[qw3rtman]

Looks good. Merging. 👍

[qw3rtman]

@JanSchulz, I really like displaying the commit body under merge commits. Could you send in a pull request making that the default behavior (in git changelog) as well?

Thoughts @hemanth?

[jankatins]

Done...