Documentation: merge: use MERGE_HEAD to refer to the remote branch

Documentation: merge: use MERGE_HEAD to refer to the remote branch

commit 57bddb11 (Documentation/git-merge: reword references to
"remote" and "pull", 2010-01-07) fixed the manual to drop the
assumption that the other branch being merged is from a remote
repository.  Unfortunately, in a few places, to do so it
introduced the antecedentless phrase "their versions".  Worse, in
passages like the following, 'they' is playing two roles.

|   highlighting changes from both the HEAD and their versions.
|
| * Look at the diffs on their own. 'git log --merge -p <path>'

Using HEAD and MERGE_HEAD nicely assigns terminology to "our" and
"their" sides.  It also provides the reader with practice using
names that git will recognize on the command line.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Thomas Rast <trast@student.ethz.ch>