Documentation/git-merge: reword references to "remote" and "pull"

Documentation/git-merge: reword references to "remote" and "pull"

The git-merge manpage was written in terms of merging a "remote",
which is no longer the case: you merge local or remote-tracking
branches; pull is for actual remotes.

Adjust the manpage accordingly.  We refer to the arguments as
"commits", and change instances of "remote" to "other" (where branches
are concerned) or "theirs" (where conflict sides are concerned).
Remove the single reference to "pulling".

Signed-off-by: Thomas Rast <trast@student.ethz.ch>