string pad: handle padding characters of width > 1

If the padding is not divisible by the char's width without remainder,
we pad the remainder with spaces, so the total width of the output is correct.

Also add completions, changelog entry, adjust documentation, add examples
with emoji and some tests.  Apply some minor style nitpicks and avoid extra
allocations of the input strings.

tests/checks/string.fish

@@ -45,10 +45,7 @@ string length -q ""; and echo not zero length; or echo zero length
 string pad foo
 # CHECK: foo
 
-string pad -r -w 4 foo
-# CHECK:  foo
-
-string pad -r -w 7 -c '-' foo
+string pad -r -w 7 -c - foo
 # CHECK: foo----
 
 string pad --width 7 -c '=' foo
@@ -57,14 +54,36 @@ string pad --width 7 -c '=' foo
 echo \|(string pad --width 10 --right foo)\|
 # CHECK: |foo       |
 
+# Pad string with multi-width emoji.
 string pad -w 4 -c . 🐟
 # CHECK: ..🐟
 
+# Pad with multi-width character.
+string pad -w 3 -c 🐟 .
+# CHECK: 🐟.
+
+# Multi-width pad with remainder, complemented with a space.
+string pad -w 4 -c 🐟 k kk
+# CHECK: 🐟 k
+# CHECK: 🐟kk
+
+# Pad to the maximum length.
 string pad -c . long longer longest
 # CHECK: ...long
 # CHECK: .longer
 # CHECK: longest
 
+# This tests current behavior where the max width of an argument overrules
+# the width parameter. This could be changed if needed.
+string pad -c_ --width 5 longer-than-width-param x
+# CHECK: longer-than-width-param
+# CHECK: ______________________x
+
+# Current behavior is that only a single padding character is supported.
+# We can support longer strings in future without breaking compatibilty.
+string pad -c ab -w4 .
+# CHECKERR: string pad: Padding should be a character 'ab'
+
 string sub --length 2 abcde
 # CHECK: ab
 
@@ -644,5 +663,5 @@ echo $status
 # CHECK: 0
 
 # Unmatched capturing groups are treated as empty
-echo 'az'   | string replace -r -- 'a(b.+)?z' 'a:$1z'
+echo az | string replace -r -- 'a(b.+)?z' 'a:$1z'
 # CHECK: a:z