Add string comparison functions that respect the ignore_case variable.

Add string comparison functions that respect the ignore_case variable.

Multiple locations within this patch series alter a case sensitive
string comparison call such as strcmp() to be a call to a string
comparison call that selects case comparison based on the global
ignore_case variable. Behaviorally, when core.ignorecase=false, the
*_icase() versions are functionally equivalent to their C runtime
counterparts.  When core.ignorecase=true, the *_icase() versions perform
a case insensitive comparison.

Like Linus' earlier ignorecase patch, these may ignore filename
conventions on certain file systems. By isolating filename comparisons
to certain functions, support for those filename conventions may be more
easily met.

Signed-off-by: Joshua Jensen <jjensen@workspacewhiz.com>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>