checkout --conflict=diff3: add a label for ancestor

checkout --conflict=diff3: add a label for ancestor

git checkout --conflict=diff3 can be used to present conflicts hunks
including text from the common ancestor:

	<<<<<<< ours
	ourside
	|||||||
	original
	=======
	theirside
	>>>>>>> theirs

The added information is helpful for resolving a merge by hand, and
merge tools can usually understand it without trouble because it looks
like output from ‘diff3 -m’.

diff3 includes a label for the merge base on the ||||||| line, and it
seems some tools (for example, Emacs 22’s smerge-mode) cannot parse
conflict hunks without such a label.  Humans could use help in
interpreting the output, too.  So change the marker for the start of the
text from the common ancestor to include the label “base”.

git rerere’s conflict identifiers are not affected: to parse conflict
hunks, rerere looks for whitespace after the ||||||| marker rather
than a newline, and to compute preimage ids, rerere has its own code
for creating conflict hunks.  No other code in git tries to parse
conflict hunks.

Requested-by: Stefan Monnier <monnier@iro.umontreal.ca>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>