Do not load user or system gitconfig and gitattributes

While doing any kind of git loading, unset HOME variables and set
NOSYSTEM variables so that cgit does not load any settings that a user
may have set for his own /usr/bin/git usage.

This fixes a fatal error introduced with git 1.8, whereupon git would
fatally exit when failing to access particular files.

The result of this is that only repo-local configuration files are
accessed:

zx2c4@thinkpad ~/Projects/cgit $ HOME=/root QUERY_STRING="url=foo/log"
CGIT_CONFIG=tests/trash/cgitrc strace -e access ./cgit >/dev/null
access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory)
access("repos/foo/.git/objects", X_OK) = 0
access("repos/foo/.git/refs", X_OK) = 0
access("repos/foo/.git/config", R_OK) = 0
access("repos/foo/.git/config", R_OK) = 0
access("repos/foo/.git/objects/b3/bafdbf0183f4897ef8b1319cb8c490ed54717e", F_OK) = 0
access("repos/foo/.git/objects/b3/bafdbf0183f4897ef8b1319cb8c490ed54717e", F_OK) = 0
access("repos/foo/.git/objects/b3/bafdbf0183f4897ef8b1319cb8c490ed54717e", F_OK) = 0
access("repos/foo/.git/objects/b3/bafdbf0183f4897ef8b1319cb8c490ed54717e", F_OK) = 0
+++ exited with 0 +++

Reported-by: Ferry Huberts <ferry.huberts@pelagic.nl>
Tested-by: Jason A. Donenfeld <Jason@zx2c4.com>
Tested-by: Ferry Huberts <ferry.huberts@pelagic.nl>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>