am: counteract gender bias

am: counteract gender bias

Since 47f0b6d5 (Fall back to three-way merge when applying a patch.,
2005-10-06), i.e. for almost 11 years already, we used a male form
to describe "the other tree".

While it was unintended, this gave the erroneous impression as if
the Git developers thought of users as male, and were unaware of the
important role in software development played by female actors such
as Ada Lovelace, Grace Hopper and Margaret Hamilton. In fact, the
first professional software developers were all female.

Let's change those unfortunate references to the gender neutral
"their tree".  Doing so also makes the fallback_merge_recursive(),
which is an oddball, more in line with the other parts of the system
where we contrast what we have vs what we obtain from others by
saying "ours" vs "theirs".  This inconsistency was also unintended.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>