Fix several issues in cmd_ac_complete_filepath:
* Prevent a segfault when input is empty.
* Fix a double free where acstring was managed by both auto_gchar and GArray.
* Fix a memory leak when reassigning inpcp during quote stripping.
* Restore the ability to cycle through files on repeated TAB presses by
caching the last input and skipping updates if the input is already a
known completion.
* Preserve user input style (e.g. ~, ./) in autocompletion strings to
ensure matches are correctly displayed and filtered.
Bug got introduced when "cleaning" code with new compiler flags and
sanitizers: aec8e48268.
Add unit tests so this doesn't happen again.
Before that commit we didn't use the static variable but used
autocomplete_update instead. Now we avoid redundat updates and preserve
the state across tab presses. We should look at this again later.
Fix https://github.com/profanity-im/profanity/issues/2098
8dbd1e3253