Add QuotedString class to handle Git path style quoting rules

Add QuotedString class to handle Git path style quoting rules

Git patch files can contain file names which are quoted using the
roughly the C language quoting rules.  In order to correctly create
or parse these files we must implement a quoting style that matches
those specific rules.

QuotedString itself is an abstract API so callers can be passed a
quoting style based on the context of where their output will be
used, and multiple styles could be supported.  This may be useful
if jgit ever grows a "git for-each-ref" style of output where Perl,
Python, Tcl and Bourne style quoting might be necessary.

References through the singleton QuotedString.GIT_PATH should be
able to bypass the virtual function table, as the specific type is
mentioned in the field declaration and that type is final.  A good
JIT should be able to remove the abstraction costs when the caller
has hardcoded the quoting style.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Robin Rosenberg <robin.rosenberg@dewire.com>