filemerge: split internal merge into api entry point and internal helper
This is a step toward adding 'union merge' to the internal merge tool.
'union merge' is a merge strategy which adds both left and right hand side
of a conflict region. Git implements this merge strategy which is very
practical to have for merging to e.g. the Changelog file.
#require execbit
$ hg init repo
$ cd repo
$ echo foo > foo
$ chmod 644 foo
$ hg ci -qAm '644'
$ chmod 755 foo
$ hg ci -qAm '755'
reverting to rev 0
$ hg revert -a -r 0
reverting foo
$ hg st
M foo
$ hg diff --git
diff --git a/foo b/foo
old mode 100755
new mode 100644
$ cd ..