git-svn.txt: elaborate on rev_map files

git-svn.txt: elaborate on rev_map files

The man page for `git svn` describes a situation in which "'git svn'
will not be able to rebuild" your $GIT_DIR/svn/**/.rev_map* files, but
no mention is made of in what circumstances `git svn` *will* be able to
do so, how to get `git svn` to do so, or even what these files are.

This patch adds a FILES section to the man page with a description of
what $GIT_DIR/svn/**/.rev_map* files are and how they are (re)built, and
links to this description from various other parts of the man page.

Signed-off-by: Keshav Kini <keshav.kini@gmail.com>
Signed-off-by: Eric Wong <normalperson@yhbt.net>