Config: Drop backslash in invalid escape sequences in subsections

Config: Drop backslash in invalid escape sequences in subsections

These are ignored by C git when parsing:

$ git config -f - --list <<EOF
[foo "x\0y"]
bar = baz
[foo "x\qy"]
bar = baz
[foo "x\by"]
bar = baz
[foo "x\ny"]
bar = baz
[foo "x\ty"]
bar = baz
EOF
foo.x0y.bar=baz
foo.xqy.bar=baz
foo.xby.bar=baz
foo.xny.bar=baz
foo.xty.bar=baz

This behavior is different from value parsing, where an invalid escape
sequence is an error (which JGit already does as well):

$ git config -f - --list <<EOF
[foo]
bar = x\qy
EOF
fatal: bad config line 2 in standard input

Change-Id: Ifd40129b37d9a62df3d886d8d7e22f766f54e9d1