storageutil: new function for extracting metadata-less content from text
Other storage backends will want to do this.
I'm not concerned about Python function call overhead because I
expect self.revision() to dwarf the function call overhead time,
since self.revision() requires multiple function calls and may
involve decompression in the common case.
Differential Revision: https://phab.mercurial-scm.org/D4756
$ hg init
$ touch unknown
$ touch a
$ hg add a
$ hg ci -m "1"
$ touch b
$ hg add b
$ hg ci -m "2"
Should show unknown
$ hg status
? unknown
$ hg revert -r 0 --all
removing b
Should show unknown and b removed
$ hg status
R b
? unknown
Should show a and unknown
$ ls
a
unknown