color-identifiers-mode.el: limit (looking-back) with line beginning

(looking-back) function has bad performance, which is explicitly
documented. So it's better to limit its use, in general, or at least to
a portion of buffer.

In my tests, search operations on Python buffers resulted in big CPU
load, which I tracked down to (looking-back) function. Limiting
the call with beginning of the buffer improved time a lot.

For testing I searched not_found_message in interpetator.py file of
meson, and measured time of (color-identifiers:scan-identifiers)
function.

Before:

(0.000129431 0 0.0)
(0.001869894 0 0.0)
(0.001798768 0 0.0)
(0.001853524 0 0.0)
(0.002508738 0 0.0)
(0.005276375 0 0.0)
(0.004376699 0 0.0)
(0.004512336 0 0.0)
(0.004265427 0 0.0)
(0.008791819 0 0.0)
(0.00790308 0 0.0)
(0.549390703 0 0.0)
(0.330451611 0 0.0)
(0.48631925 0 0.0)
(0.032972452 0 0.0)
(0.273051493 0 0.0)
(0.104208415 0 0.0)

After:

(8.4067e-05 0 0.0)
(0.001115394 0 0.0)
(0.000975669 0 0.0)
(0.001217772 0 0.0)
(0.002791122 0 0.0)
(0.002674071 0 0.0)
(0.002754122 0 0.0)
(0.001712691 0 0.0)
(0.000947949 0 0.0)
(0.000735731 0 0.0)
(0.001101217 0 0.0)
(0.001080936 0 0.0)
(0.000826694 0 0.0)
(0.000825475 0 0.0)
(0.000113389 0 0.0)
(0.001007357 0 0.0)
(0.001217875 0 0.0)

Signed-off-by: Konstantin Kharlamov <[email protected]>

color-identifiers-mode.el

@@ -766,7 +766,7 @@ evaluates to true."
                              (and flface-prop (memq flface-prop identifier-faces)))
                            (get-text-property (point) 'color-identifiers:fontified)))
                   (goto-char (next-property-change (point) nil limit))
-                (if (not (and (looking-back identifier-context-re nil)
+                (if (not (and (looking-back identifier-context-re (line-beginning-position))
                               (or (not identifier-exclusion-re) (not (looking-at identifier-exclusion-re)))
                               (looking-at identifier-re)))
                     (progn