[TASK] Skip GeneralUtility::resolveBackPath() if its not necessary

resolveBackPath() is run a couple hundred times per script run,
especially in the backend. As this method is only really necessary in a
few special cases (when a path really contains "../"), we can skip it
for all other paths with an early return.

The check is really weak (it will not skip the call for things like
"/some/dir..with..dots..inbetween/"), but this is ok because we just use
it to skip all *really* obvious cases while at best not missing *any*
legitimate use-case, which should be achieved with this commit.

Change-Id: I4e7fbd2076323e682fcf66bc57605f817cd3e03c
Resolves: #55519
Releases: 6.2
Reviewed-on: https://review.typo3.org/27197
Reviewed-by: Jan Helke
Tested-by: Jan Helke
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn