support fetching into a shallow repository

support fetching into a shallow repository

A shallow commit is a commit which has parents, which in turn are
"grafted away", i.e. the commit appears as if it were a root.

Since these shallow commits should not be edited by the user, but
only by core git, they are recorded in the file $GIT_DIR/shallow.

A repository containing shallow commits is called shallow.

The advantage of a shallow repository is that even if the upstream
contains lots of history, your local (shallow) repository needs not
occupy much disk space.

The disadvantage is that you might miss a merge base when pulling
some remote branch.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>