context: convert to hex for error message only for 20-byte changeid
Now that 20-byte strings unambiguously mean binary (or a bug), we can
specialize the conversion to hex for that case.
Differential Revision: https://phab.mercurial-scm.org/D3452
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls
a