hgtagsfnodes: refactor code to compute fnode into separate fn
I plan to use this code at one more place while fixing a bug caused by an
invalid fnode present in cache.
Differential Revision: https://phab.mercurial-scm.org/D10013
error: remove shortening of node in error message
This strips the complete 20 bytes node which was not found. Having the the full
node in error message is important as it makes debugging easier.
If a short node is to be displayed, that should be done by callers.
Differential Revision: https://phab.mercurial-scm.org/D9994