lib/repo: Allow disabling lock timeout

lib/repo: Allow disabling lock timeout

Currently the locking code checks if the value -1 was set for the config
key "lock-timeout-secs" and if so, a thread trying to acquire a lock
will block indefinitely. Positive values specify how long to attempt to
acquire a lock in a non-blocking way (the attempt is made once every
second). But when the value is read from the config file,
g_ascii_strtoull() is used, which converts it to an unsigned integer.
This commit makes libostree use g_ascii_strtoll() instead, so that it's
possible to set that key to -1 as intended.

Closes: #1737
Approved by: jlebon