git-gui: Remove usernames from absolute SSH urls during merging

git-gui: Remove usernames from absolute SSH urls during merging

If we are being asked to merge a tracking branch that comes from a
remote repository accessed by the very common SSH URL format of
"user@host:/path/to/repo" then we really don't need the username
as part of the merge message, it only clutters up the history and
makes things more confusing.  So we instead clip the username part
off if the local filesystem path is absolute, as its probably not
going to be an ambiguous URL even when it is missing the username.

On the other hand we cannot clip the username off if the URL is
not absolute, because in such cases (e.g. "user@host:myrepo") the
directory that the repository path is resolved in is relative to
the user's home directory, and the username becomes important.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>